IE8 comes with an "Emulate IE7" button, which will revert the page back to run as IE7. This is great for developers who can quickly test their apps in both versions (I just wish we could have an IE6 button too!).
I ran some few tests on the fresh IE8 beta1. It still looks pretty much like IE7. This blog runs with only one minor problem. I have a gallery page that relies on AjaxControlToolkit, and this really broke on IE8. Hopefully the AjaxControlToolkit team will quickly get all those issues fixed.
There are 4 IE8 specific sessions at the Mix conference. Keep an eye on them at the Mix Website when they come online (about 24 hours after the session):
BCT08 - Welcome to Internet Explorer 8 Wed March 5 at 1:30 PM to 2:45 PM
CT07 - Cross-Browser Layout with Internet Explorer 8 Wed March 5 at 3:00 PM to 4:15 PM
T21 - Integrating Your Site With Internet Explorer 8 Thursday March 6 at 8:30 AM to 9:45 AM
T04 - Developing Cutting Edge Web Applications With Internet Explorer 8 Friday March 7 at 8:30 AM to 9:45 AM
Here's a picture from the first session, showing the Acid2 test: