Since I've already been described as a Microsoft apologist, despite creating delightful little films to mock their products, I might as well point out something I think was overlooked in the launch of Windows Vista and Microsoft Office 2007: A great new font!
It's Segoe UI, which is bundled with the new versions of Microsoft's flagship OS and productivity suite. And like the Microsoft TrueType core fonts, Segoe could very well present an interesting new option for web developers.
Of course, the usual caveats apply. It'll be a while until the font is in common circulation, and it might never make it onto other platforms like the Mac and Linux. But that's why we have a set of fallback fonts; If you know that some large percentage of your audience might have access to the font, or you're in a controlled environment like an intranet, it's nice to have another good option.
Microsoft's System font information page for Vista has some additional good info on usage and guidelines.
(For what it's worth, I only blog about Microsoft regularly because it seems like there's very little discussion that isn't either shameless fanboyism or pointless bashing. I'll take another font option in my CSS from anybody that can offer it to me.)
Thank god for a new web font... our options on the web were getting extremely limited!
Getting on my Mac wasn't a problem - I just copied it from my Windows Vista partition :) . But you're right - while it looks nice, there's no point in requiring it for anything until it becomes more available.