Thanks to Michael Adams AJAX is featured in more places in the WordPress admin. We use AJAX primarily to avoid page reloads when manipulating lists. When deleting posts, pages, categories, comments, or bookmarks, you will see the deleted object fade out of the list and vanish. No reload required. We do this in a few places in 2.0.x, but now we do it more comprehensively. See this tracking ticket for details. This change will be available in the next release of WordPress. Right now you can get it only by pulling the latest development bits out of the subversion repository.
And, yes, the admin pages degrade nicely when using less capable browsers. I can still administer my blog using my BlackBerry.