If they screw something in Your N900 and deny to take responsibility, it may be hard for You to proof that it's they fault. I have experiences like this with repairing not working backlight in notebook - should be easy cause it's common problem - for me, it was broken tape-like cable, shortcutting something. But idiot in repair shop wanted to replace inverter on motherboard, and to check if it's not fuse problem, he replaced it (fuse) with wire, really frying inverter. At last, on 3th repair shop i find someone capable of diagnosing properly and repairing everything. Also he was the one who discovered responsible flat multicable, and wire in place of fuse. Still, it costed me 100 $ more, cause fried inverter - that was perfectly fine before idiots in 1st or 2th shop put their hands on it.

I would not risk allowing any "technican" who seems to not even diagnose issue properly, to go "intimate" with my N900.