Dont't know about welding, but there is somewhere a manual in this forum on how to glue it before it breaks. There are three N900 in our household two with broken usb. we use them with seperate chargers you'll find them on ebay for less than 2€. What i did with my device is, i used sugru (a kind of trademarked silicon that you can use like modelling clay) to fasten the usb before it could start breaking off. There is actually a lot of space around the port. i filled it all up with this silicon. Seems to work for more than a year now.
Soldering is very hard to do i belive, though i am not an expert at all, because it is hidden under a part of the camera i belive.