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.)