I've met the same problem as a number of other users with 2.8/2.8.1 : everything Ajax stopped working on the admin pages.
After looking around a bit, I tried deactivating all my extensions, and lo and behold, one of them was indeed responsible for that problem.
In my case, it was Category Icons. A fix already exists here : http://www.category-icons.com/2009/06/category-icons-patch-for-wordpress-28/.
Hope this helps someone somewhere :)