I noticed a post last week on the Microsoft XML Team's blog.
Apparently they are going to disable the use of MSXML version 4 in Internet Explorer, sometime after October, for security reasons amongst other things. They have already advised people to use either version 3 or version 6 in another earlier post. MSXML is used heavily by Ajax developers to make background requests to web services whilst the user interacts with the page.
This could mean quite a few sites not working later this year. Although most sites use a trial and error method of instantiating the XmlHttp class, which means they try with the higher version and work down until one works, I personally know of a few intranet sites that knew the desktops had version 4 installed and so just used that.
Anyway I expect there to be a flurry of postings in the newsgroups come autumn.