I haven't tried it out personally, but over on the Habitually Good blog, Vaibhav says that Google's new Chrome browser doesn't work with Windows Speech Recognition in Windows Vista. That's too bad... Both IE7, IE8 Beta 2, and Firefox work great with it.
Hey Google: What's up? If any Google Devs want to learn more about how to add support for SR, let me know. I'll point you to the documented interfaces for doing that.