I'm a Mac user so I only know by hearsay that IE still doesn't have tabbed browsing.
It's literally been years since I would even consider using a browser without tabbed browsing. When you commonly have as many Web sites open at once as I do, it makes a huge difference. (Especially OmniWeb's implementation.)
I can't imagine why Micrsoft wouldn't add this simple, useful feature to their browser. If a user doesn't like it, they don't have to use it. No harm is done by its presence. Even if it is argued that not everyone like the tabbed browsing, that's no excuse for not providing it as something that may optionally be switched on in the Preferences pane.
I don't think Microsoft can argue that they don't have the engineering budget, given that every other real browser, built by volunteers or by companies large or small, has the feature.
The thing that is really stunning to me is that Microsoft appears to be betting that Longhorn will have enough highly user-friendly features that it will motivate people not to switch to Linux. But if a company with the budget similar to that spent to get us to the moon can't avoid being the very last one to implement tabbed browsing, one really has to wonder.