Jan, the developer of Gravity, mentioned a few times on Twitter that he was thinking of porting it to Maemo 5. No commitments though.

I too would love to see a full-featured Twitter client for the N900. Would make sense too given the geeky audience of this device. The lack of a good Twitter client is one of the main reason why I didn't stick with the N810 very long.

Tried Mauku on the N810 but although it did look quite nice, it was really too limited compared to the other mobile Twitter clients I got used to. Still looking forward to see what's it's going to be like on the N900.