Ah, the Famous Byron the Bulb... I'm afraid I'll have to disagree with StrawberryFrog's statement that in writing about The Bulb, Pynchon is just taking up space. Like many great books, Gravity's Rainbow provides a reading experience a bit similar to taking an LSD trip (or living appx. 1 day with ADD): you suddenly discover dimensions of everything that weren't visible before, and for that matter, you suddenly discover that everything has more dimensions than you thought were possible... in fact, more dimensions than you can imagine.

Byron the Bulb's story can be taken as...
* a treatise on planned obsolescence, but with the bizarre element of the product planned for obsolescence being an individual, a ghost in the machine. But it's more poignant (chilling?) than that: the Bulb is, despite its immortality, powerless -- this kind of reminds me of Terry Gilliam's Brazil.
* a reflection of the anthropomorphized bulbs appearing in head comics of the period (this is not as ludicrous as it sounds when you consider the eery resemblance between "The Counterforce" and hero teams like the Fantastic Four... hell, the Counterforce even had a Plastic Man.)
* the symbology of light bulbs as ideas (keep in mind that Byron hovers over the heads of several characters in the book). There could even be a hint of guardian angels or of halos, though considering that BtB hovers over a deadly amphetamine-addled barber, this would take some explaining.
I will leave the discovery of other connotations and connections as an exercise for the reader.

And if the ending is a bit of a downer... it should be: after all, the trajectory of the V-2 was a parabola...
As an illustration of the "musical" or "operatic" nature noted above, I'll note my own experience: I found myself thinking up tunes just to go with the many "lyrics" in the novel. "Who'd ever think-it could start such a flap?..."
Random note: The mythical late-war and post-war Europe of GR strongly reminded me of Hakim Bey's Temporary Autonomous Zones.