eWeek.com reported yesterday that screenshots of IE7 have been posted on the web, and also for a limited time a download of beta code (although this was later removed).
Major changes in the browser are security enhancements and the addition of tabs for browsing multiple pages, similar to what Opera and Firefox have been providing for number of years now (I remember seeing tabbed browsing in Opera back around 2001/2002 and thought it was an awesome feature).
Seems like MS feels a need to keep up with the Joneses.