My apologies if this seems obvious and you've already played with this option, but one never knows... When you tap on the map and it brings up the little gear icon (edited - actually I now believe this icon is supposed to be an eye, since it is brings up the "View" option menu - haha) on the bottom of the screen, then you choose that icon, do you have "Auto-Center" set to something besides "None"?
With Linux-powered N900..ANYTHING is possible