Finally making time to post some thoughts about this.
Wow! Thank you Microsoft. Now we have FF, Opera, and IE8 as platforms for accessible web2.0 applications. Notice any browser missing? (See end)
Don't read it please, but the MSDN has an article about support for ARIA markup in IE8.
Now, this is awesome, but... As Anne van Kesteren points out the good news is tainted by the non standard way ARIA information can be accessed in IE's DOM. I'm not going to give an example here since I don't want to accidentally promote potential IE only accessibility syntax where cross platform syntax exists.
We're on the case and are hopeful IE8 will change the docs to promote cross browser accessible DHMTL development (the setAttribute/getAttribute way).
WebKit where is your ARIA support? My nagging continues.
[Updated MSDN link]