If it really is only a problem with newer 3 USIM cards I'd imagine they've changed supplier. There are lots of fancy things that can go on there these days (Java SIMs, etc) and tools to allow better over the air management, I'd guess Three are using something new. I believe it's not every countries version of three either -- I think I read three Italia cards work fine.

Since I've a 3 USIM which is a few years old I'm hoping it'll work, but I'm holding of any N900 purchase until I'm sure.