Right now: not a chance. While the Symbian^3 code was indeed dump^Wreleased this month, it's still far from being a useable system. See http://blog.symbian.org/2010/02/05/i...n-so-now-what/ for details (high-level summary: doesn't compile with GCC, slow/buggy graphics).

In the longer term: maybe (I suspect Symbian developers might end up using N900s as developer "boards"), but why would you want to? Symbian^3 won't run existing S60/UIQ apps, and the Qt apps it will run are supposed to be easily portable to Maemo.