I've had problems with this too. I reckon the phone doesn't charge over USB because the screen is always on (as mentioned in this post earlier) when connected to active sync. Thing is, if you turn the screen off (using power button) then the phone disconnects from active sync and stops charging! It's a no win situation.

I've got round this by installing a programme called ActiveSyncToggle on the PC. It allows you enable/disable active sync. With active sync disabled the phone will charge over USB because the screen dosn't come on when connected to the computer. Of course, the phone dons't sync, but for me that's not an issue as i prefer to sync over GPRS anyway....

Hopefully some of you guys will find this useful.