Quick reply.
Try Ovi Suite, version 2.2
http://europe.nokia.com/support/down...-text-messages
Both Nokia PC Suite and Ovi Suite can work with N900 (in USB PC Suite Mode), just make sure that only one of these two programs is running on the PC (otherwise, the two suites will fight with each other)

However, I'm afraid that neither of them supports sending/receiving messages yet. I don't know. Ask Nokia.