I was reading E2 and at some point I reached the bottom of a page and E2 said to me, "Y'know, if you login, you can write something here. You can also Create a New User if you don't already have an account." Since I have no willpower, I did end up logging in, but not before taking a moment to ponder the word login. It seemed out of place where it was.
I've always thought of login to be a noun and log in to be the verb. The logic behind this, I suppose, is that in log in, log is the verb and in is where you're logging, and login is just some word a guy in thick glasses made up for the thing people are supposed to type when they are logging in (i.e., "Please type your login and password in the boxes below."). Turns out I'm pretty much correct! Suck it, bitches!
Webster 1913 has nothing to say on the topic, which is kind of surprising since it was published only eighty or so years before computers became a big thing. The Internet, however, gave me a whole bunch of definitions for login. They all were some variation of "the process of authenticating yourself with a computer. Noun!" So while I was wrong about the definition (a login is the process of logging in, not the thing you type when you log in), I was right about the whole noun/verb bit. Which is more important in the long run.
So, in case anybody was wondering, log in is always a verb (or, if you want to be anal about it, a verb and a preposition) (update: DonJaime says it is a phrasal verb! Thanks, guy!), and login is always a noun. This is what the Internet tells me, anyway. I could be wrong, and with language being ever-evolving or whatnot, it probably doesn't matter anyway.
On the bright side, if you're ever on a date with a cute guy/girl, you can bust out this factoid about the difference between log in and login. One time I spent five minutes straight talking to a girl about the difference between they're, there and their. Next week, we'll have been married five years.*
*Not true