You can get some info here:
http://cmd.nokia.com/phoenix.zhtml?c...&p=irol-hosted

Basically the N900 is a technology enthusiast gadget that will be fully integrated into the Ovi market and Qt/web during a lifespan of 2 years (or more?). Looks to me there will be more Maemo5 devices coming before a "native" Maemo 6 device will ship in a couple of years.