If you can find a place that reflow things for PS portable (or other, similarly sized pieces of electronic), they should be able to do it for N900. you must be able to tell them which chip exactly have broken solder joints.

Other than that, roaster oven reflow in home is only solution. I have one N90 0with exactly this problem, that is used by my son (almost 6 years old, he doesn't need SIM functionality yet, but broken modem breaks GPS, too...), and I'll try to do this repair as soon as I get a fully working replacement for him. Just for the sake of curiosity, baking N900 in home oven sounds like lot of fun for whole geeky family

/Estel