I don't know why I didn't think of trying this before.. but I just created a new skype account and added my real account as a friend. When I log in on the n900 with the new account I can see myself and the sound test service ok in the address book and when I go to "new im". The new account seems to work fine, so maybe the n900 has decided it doesn't like one of my friends names anymore on my real account and it is causing the whole friends list to be unreadable.