Depends on how you define big though. Its just as capable as PHP of scaling to larger sites because it scales in exactly the same way, using share-nothing architecture. There is a large mortagage processing application written in RubyOnRails but the name escapes me (its in the Agile Rails book). We've also got some fairly large apps planned for a huge retail employment company in the UK.
Either way, I'm less concerned about how many "big sites" are using it, and more concerned about how productive and enjoyable development can be for me.
I have to agree on the Windows front though. I wouldn't ever recommend it as a platform for deploying Rails apps (though I wouldn't recommend it for deploying PHP apps either, not when you can deploy to a *nix platform) and its not the nicest development environment either - its not overly difficult to get up and running for development purposes though. Installing Ruby and RubyGems is a sinch using the one-click installer then installing Rails using gems is as easy as it is on *nix. I don't enjoy developing in Windows as much though, thats why I'm getting myself a MacBook Pro soon