This has happened to me twice now so I figured it was worth adding a topic, if not for my own future resolution.
If you own an Android HTC 3G phone, you already know that it comes with a weird non-trapazoidal USB cable/connector. When I went to Radio Shack to pick up a spare cable, the woman there told me that you could use a normal mini USB cable to charge it, which I did and sure enough the phone did charge using the generic cable. But here's the rub; the generic mini USB cable does NOT allow you to connect/sync the phone to your computer and produces the evil "device not recognized" message.
The moral: ALWAYS USE THE NASTY SPECIAL CABLE WHEN SYNCING YOUR ANDROID 3G PHONE TO YOUR COMPUTER.
While I am grateful that HTC had the foresight to allow the phone to be charged from a generic mini-USB, it is counter-intuitive, and frankly, suspicious, that the computer cannot connect with it. I suspect is it somehow related to one of the other bitches I have with the phone which is its lack of a mini headphone jack. It comes with a little adapter that plugs into the boot shaped would-be mini usb connector with a headphone jack on the other (gee, how inconvenient could that be?). Clearly they added extra pins for audio but for reasons unknown have broken the standard USB wiring. It's probably best to use generic cables when connecting to a charger and using the official cable when connecting it to the computer in case HTC decided to blast audio into one of data pins which could be a very bad thing.