Or a link to the Nokia RX-51/N900 Service Manuals for detailed information on disassembly; combine that with whichever approach to the USB port fix takes your fancy. The L1/L2 service manual should hopefully give some confidence that it's not rocket science and will be doable with a bit of care.

I'm about to do this on one, maybe two N900s, and think I favour using a thin "worm" of epoxy putty around the sides and the back of the port, reinforcing the pads and at the same time filling in the space under each side of the USB port. In theory (I haven't done it yet), this should adhere to more surface area than solder and will physically fill up the triangular prism shaped spaces under each side of the jacket to provide a mechanical buttress against movement. We shall see.

Update: as of September 2011, the epoxy putty in question is available for £2.19 delivered via eBay, a precision screwdriver set containing PH0 and T6 bits (among others) in a pen-style barrel is £4.00 delivered, and a pair of plastic pry tools - straight and angled - is 99p delivered. I mention this for the benefit of those lacking the confidence to pull their beloved N900 apart and find out what's inside - the knowledge you need is contained in the L1/L2 service manual plus the guides on this forum, and the tools you need come to all of £7.18, incl. delivery.