One of the reasons this blog has been so quiet over the past few, uhm, months is that I've been spending a considerable amount of my time working with a few others on the "Browser UI" team to implement this feature. Expect some more technical posts soon.
If you have questions or topic ideas please post them and I'll answer what I can. Tabbed Browsing has required a lot of work on the internals and "application-level" features in IE, so that's where I can provide the most useful info. Contrast this with the HTML rendering engine, which I can't say much about other than that there are a bunch of great people working in this area and the main IE blog is probably the best place to stay informed.