Volume button fell off...

When i was last at the Genius Bar, I saw someone bring a problem about the volume buttons having fallen off (but they did not take apart their phone), and he was given a replacement phone- he had the buttons that fell off in a ziplock baggie and the iPhone looked in tact . . . but you could probably explain . . . i find that the genius bar techs at the Apple Store are very helpful . . .

Sounds like a voided warranty to me, but no harm in trying to get it replaced at an Apple store.

Since you've voided your warranty by disassembling the phone, you could try a little dab of superglue to put the volume button back on.