Try asking on Nokia Forums or sites for Symbian developers, because that would be the proper place to ask if there is symbian software to use your phone as a keyboard.

The second option would be create special programs for both N900 and symbian phone, then on symbian phone you would launch it, it would send via bluetooth keypresses, send to application on N900, application on N900 would generate system event like if key on keyboard was pressed (I have no idea if that is possible system-wide).

Anyone knows if it is possible to generate system-wide keypress events programaticaly on N900? If yes, how? If I knew it, I could write a simple app for N900 and JavaME app for other phones.

Edit: OK, I found a way to simulate keypresses using xdotool I'm writing app now, I should finish later tommorow, it will work with all Java ME enabled phones with Bluetooth support.