I just removed all tasks from my XV6700 which is a Windows Mobile device. It can't handle thousands of tasks. And even though most of these tasks are marked completed, ActiveSync will not allow my to synch only active (uncompleted) tasks. Users of Palm OS have written on this forum that they have no such limitations. Blackberry users have not responded clearly one way or another.
I am seriously considering abandoning WM until they can handle power users of tasks.