Info On Tabbed Browsing in IE7

IEBlog : IE7 Tabbed Browsing Implementation
My role has been to re-architect IE to support tabbed browsing. This work began last year and includes building a new frame (top-level window and chrome), sorting out how to host and switch between multiple instances of the browser, and managing communication between the various internal components

Good insight on how tabs are being architected in IE7.