I found the Soldering advice needed for re-attaching N900 USB to mainboard thread.

By the way, I just realised how it happened in my case. It must have been the way I charged N900 in the car. I used the ProClip Mount & Holder with Tilt Swivel and I connected it to a cigarette lighter charger + Nokia CA-146C Charger Adapter. Obviously that hanging cable added too much weight to the already defective USB port and eventually forced it to break.

Instead, I should have bought the ProClip Charging Holder with Tilt Swivel and Cigarette Lighter Adapter, but that seemed too expensive...

So all of you out there that have the same charging habits beware!