Up to quite recently, my Blackberry won't connect to my computer via a USB connection. It connects for about five seconds and then disconnects and displays "disconnected".
Have you tried a battery pull or reboot your computer? My computer seems to like my BB but my computer didn't like my friends... she has the same phone and carrier etc..
I did install the new version of Desktop Manager. As I was having USB problems since I updated Desktop Manager, I downgraded back to the Desktop Manager that I was using but I'm still experiencing problems.