That was unexpected... at least to me. If you go grab the most recent version of the MSN Search Toolbar with Windows Desktop Search (ugh!), you will get the new Tabbed Browsing interface months before IE7 is released.
The bit I like best is the ability to save a group of tabs as one set. The shortcoming here is that there is only one set of "My Tabs" and you overwrite it by saving a new set. I want shortcuts to multiple groups of tabs.
Anyway, if you are still using Internet Explorer, check it out!