My personal guess is that there are two options:

1) Ovi store for N900 opens with the next firmware update, which I would think happens in 1-2 weeks. This I think is the more likely one.

2) Ovi store for N900 opens sometime during spring at the time the OVI store overhaul happens. Nokia pretty much told that the current OVI store is a temporary placeholder and the real effort has gone the complete remake that should be out H1 of 2010. Hopefully the N900 wont be without a store that long.