Ya buying the n900 in the US with 1 yr warranty is a pain in the arse. And don't even try sending it to nokia care US asking for a quote for the repair, they won't even do anything but send your phone back 10x worse than when you sent it.

It's definitely a hardware issue, specifically the antenna connections. I opened up my device and found that if I "pushed" the prongs a little up (the prongs for the antenna not the sim card), they had better contact to the left "L" looking piece of the device

ie:


However it would only work occasionally, as I noticed on that "L" piece that my prongs have caused indents into the gold plates. If you can replace those plates I'm sure the connections will work again. Sadly I "pushed" my prongs a little too much that they came off and i couldn't be hassled by soldering, so i ended up buying a new one on amazon (ha!) :P