i have just had the same problem with my second N900. Bugger stopped charging. After a lot of scratching my head I finally decided to open up the phone. Really well made and well put together I have to say. I got to the USB port and could see it had come loose on one side from the board. Pushing it down made the connection again. So I decided to fold up some of my business card and stuff it between the microSD card bit and the USB port. This pushed the USB port down on to the motherboard and kept it there

I put the phone back together and of course with the card the port doesnt line up exactly with the hole but enough so that a cable can be plugged in. The keyboard on that side is ever so slightly raised, only a few mm.

I fired up the charger and was amazed to find it worked. The phone charges and connects to the PC like new. I backed it up and living happy ever since. Typed this whole reply on it.