I've just installed
IE7 Beta 2 on my development machine to try it out and see what all the fuss is about. So far I'm pretty impressed, I love the tabbed browsing and the RSS support. If it keeps going well I'll install it at home, too.
And I just tried the Quick Tabs, and that also is very cool. Not sure how useful it will be (it gives you a thumbnail of the pages in each of your tabs so you can select them based on what you're looking at rather than on pure page titles), but it looks great.
Messenger builds a number of features on the HTML rendering engine that IE is based on (often referred to at Microsoft as 'Trident'). One reason we install these pre-release versions is to make sure we find any imcompatibility issues early and get them addressed before we or they release. For instance, the tabs are basically hosted HTML controls navigated to specific pages.
I just looked at a couple of tabs and the MSN Today popup and they both look fine, so I'm not worried (at least, not about this).