Instead of displaying the task switch dialog only on the screen the mouse cursor is on, display it on each. This should improve usability, especially in situations where some monitors are powered off, with no drawbacks.
It'll be a fine idea to have the complete lists of apps somewhere on the dialog too... A little bit like the windows (^_^) task manager dialog. For example, a litlle box under the application currently selected containing the names of all other running programms...
The list is in SVN for a while, but I'm not in favour of duplicating the task list widget on every screen (what's the point of having xinerama with switched monitor anyway?)