At Leep's suggestion, I'm just trying the latest version of Calibre (0.4.136) on my Windows 7 beta and it's going great so far!
I had no installation issues (Windows 7 was a clean install on a separate partition). I'm playing around with the Metadata features and it is so very simple.
Great job Kovid! Thanks for adding the mobi support. Once I've explored all of its features, I'll start posting about it on the Amazon forums and add it to the Visual Kindle Guide wiki.
A donation will be heading your way (though you really should charge).
