Did you ever resolve this particular issue? You identified the problem that I've been struggling with over the past month. Exactly the same issues. Blackberry couldn't resolve it. Neither could T-Mobile.
I have a Sony Vaio PCG-Z1A laptop. The computer recognizes the RIM Handheld through its automatic install feature. But the BlackBerry software simply refuses to communicate and says "Handheld Not Found."
I've already tried disabling power management in the USB Root Hub, doing five completely clean installs and uninstalls of the RIM software, reinstalling the USB drivers, etc.
The handheld connects perfectly fine to my friends' computers. Mine it just refuses to play with.