The following is a listing and short explanation of the ingredients listed on the side of a can of Mountain Dew. This, of course, includes only the ingredients that PepsiCo wishes to publicly reveal and does not delve into what, exactly might be in such things as "natural flavors".

Carbonated Water: Not much to explain. Water with carbon dioxide dissolved in it. Gives you that nice bubbly/tickly feeling you want in a soda.

High Fructose Corn Syrup: A fancy, very sweet sugar. How sweet? 75% sweeter (on the "sweetness scale"?) than sucrose which, for you non-chemists out there, is table sugar. HFCS is derived from hydrolized corn starch, and glucose in the resulting corn syrup is modified (using enzymes) into fructose. HFCS officially contains 82% "solids", 43% dextrose, 31% disaccharides, 14% fructose and 12% "other". This ingredient is followed by "and/or Sugar" by which I assume they are referring to sucrose.

Concentrated Orange Juice and Other Natural Flavors: Concentrated orange juice is exactly what it sounds like, though I don't know exactly how "concentrated" it is. Probably more than the orange juice concentrate you can buy at your local grocery, though.
Other Natural Flavors can be bascially anything. But don't be thinking that means they squeezed a bunch of lemons or zested some limes. "Natural flavors" are simply the flavor/scent chemicals which chemists have been able to isolate from various real foods. It doesn't matter if they have been thereafter sythesized in a lab or extracted directly from foods...either way, they're "natural flavors".

Citric Acid: As with any acid, the concentration is important. I have no idea how concentrated the citric acid in Mountain Dew is, but it's probably safe to say that it's relatively dilute. The Materials Safety Data Sheet for citric acid provides this relevant warning:
Causes irritation to the gastrointestinal tract. Symptoms may include nausea, vomiting and diarrhea. Extremely large oral dosages may produce gastrointestinal disturbances. Calcium deficiency in blood may result in severe cases of ingestion. Also, (c)hronic or heavy acute ingestion may cause tooth enamel erosion. Yet more reasons to not drink large amounts of Mountain Dew, I guess.
Citric acid has the formula H3C6H5O7.H2O and a molecular weight of 210.14. Its solubility is ca. 60 g/100 ml @ 20C (Anhydrous), its density is 1.542 and its pH is 2.2 (0.1 N sol). It does corrode aluminum. Its oral LD50 is 3g/kg.

Sodium Benzoate: There is a parenthetical on the can following this ingredient which explains that it "preserves freshness". Benzoic acid, which is the result of dissolving sodium benzoate in acidic conditions, is an antimicrobial agent. This means that sodium benzoate prevents microbes from drinking all your Mountain Dew before you get a chance to. Your liver uses glycine to convert benzoic acid into hippuric acid, which then passes into the urine.
Benzoic acid is more toxic to cats (LD50 = 450mg/kg) than in rodents (LD50 = >1940mg/kg). So Mountain Dew is safer for your rat than your cat. Take note of this.

Caffeine: Ah, caffeine. I think most people know really everything they need to know about caffeine, so I'll just add that its chemical formula is C8H10N4O2, and it has these alternate, less sexy names: 1,3,7-trimethylxanthine and 1,3,7-trimethyl-2,6-dioxopurine. I doubt anyone will be calling themselves a 1,3,7-trimethyl-2,6-dioxopurine addict anytime soon.
A 12 oz. can of Mountain Dew contains 54 milligrams of caffeine, and the U.S. Food & Drug Administration allows a maximum of 72 mg. of caffeine per 12 oz. serving (6mg/oz) (according to the Erowid vaults).

Sodium Citrate: Chemical formula = HOC(COONa)(CH2COONa)2.2H2O. I've been unable to discover exactly what it's use is as a food additive, but it has a use in chemistry as a buffer. So it's possibly preventing your Mountain Dew from reacting/decomposing itself into some other, less marketable soft drink. Sodium citrate has another use as an anti-coagulant employed during blood plasma transfusions and/or collection.

Gum Arabic: An emulsifier. For those of you unfamiliar with emulsions (mayonnaise is another food emulsion), an emulsifier essentially holds your food product together so it maintains an appetizing texture, flavor and appearance.
Gum Arabic is also known as acacia gum, and it is collected from Acacia Senegal trees. So yes, folks, it's natural. A wholesome, natural emulsifying agent direct from Mother Earth to your can of Mountain Dew. You cannot blow bubbles with gum arabic.

Erythorbic Acid: Formula = C6H8O6. Erythorbic acid is an antioxidant. There're so many goddamn chemicals floating around in your Mountain Dew can, that you gotta keep 'em from oxidizing the hell out of each other (and the aluminum can).

Calcium Disodium EDTA: This is officially known as a "sequestering agent", and its role is to "preserve flavor". EDTA stands for Ethylenediaminetetraacetate. Say that ten times fast, and you'll understand why they just call it EDTA.
Again, it's a chemical which basically prevents the other chemicals from reacting and changing the flavor of your Mountain Dew.

Brominated Vegetable Oil: Mmm. Oily soda. The best description I could find of this is that it's a bromide ester of vegetable oil. Think Wesson plus bromine. What it does is keep your soda fizzy and flavorful, 'cause it's less polar than water. Or so I've read.

Yellow 5: The infamous Yellow Dye #5. FD&C (food dye and coloring) #5. An artificial coloring that provides you with the iconic Mountain Dew color. Yellow 5 is derived from coal tar, so in a weird way it's "natural". It's technically known as tartrazine, and there is suspicion that it can exacerbate asthma, in addition to being carcinogenic (thyroid tumors) and mutagenic. And I'm sure we're all aware of its possible role in making male Dew drinkers sterile. Food dyes have also been associated (in high doses) with learning disabilities. Coal tar is not healthy.

So. That's what's officially in Mountain Dew. Drink up, go snowboarding, bungee jumping, or do some other XTreme activity. Alternatively, quaff a can so you can stay up writing that paper. It's legal speed, folks. Yum.

It has been pointed out to me that Canadian Mountain Dew contains no caffeine. I say "fuck that shit". Down here in the US of A we're hardcore about our Mountain Dew.