Interesting,
I used a 6% isopropyl alchohol, 94% water wipes to clean mine, worked great on the exterior.
I also used it on the keyboard and within a few minutes of use I lost the paint off the most used keys. I think it must have softened the paint in some way; but maybe it was just my device. Keyboard replacements are very cheap thankfully.

I know this was an old thread...but I just came across this article from tehkseven about disassembling your N900. Perhaps reading this could be helpful and caution for anyone attempting to "open" your N900.

Nokia N900 Disassembly
http://www.tehkseven.net/news/nokia-...0-disassembly/

High End Nokia N900 Uses Cheap Screws
http://www.tehkseven.net/news/nokia-...-cheap-screws/

How To Fix The Nokia N900’s Loose Slider
http://www.tehkseven.net/news/video/...-loose-slider/

Nokia N900 Drop Tests
http://www.tehkseven.net/news/nokia-...00-drop-tests/

Nokia N900 Pocket Tests
http://www.tehkseven.net/news/nokia-...-pocket-tests/

Nokia N900 Dust Test
http://www.tehkseven.net/news/nokia-...900-dust-test/