They'd previously said 2H10?
We know Maemo 6 is coming
We know capacitive is here in these new models (and M6 is adding it)
with all the mention of the iphone, nexus one etc
my completely uneducated guess

- It's announced sometime mid year
- Nokia decides to support M6 on N900
- it starts shipping early fall
- it's not with networks/really available until end/year
- it's keyboadless, larger screen than N900
- next keyboard device isn't until mid 2011

Just a guess...