Here's a great interview with Gary Schare from Microsoft. I've talked to Gary several times and appreciate that Microsoft is turning attention to third-party developers (like me) who built tabbed browsers long before FireFox (NetCaptor in 1998).
Here's my favorite quote:
We've looked at whether you can add tabs through a browser helper object or some other way of extending IE, and it turns out you can't.
Bingo. If I it was possible to add tabs to IE via a BHO or other addon, I never would have written NetCaptor.