Can developers use iCloud in Windows apps?
Access to iCloud is controlled by Apple in such a way that each app that accesses it has to be signed and/or provided a key from Apple. That’s the easiest way to put it without violating the NDA, although the mechanism is not exactly “signing and keys”.
At the moment Apple is only providing this for OS X and iOS apps, so effectively no - Windows, web, and any other apps outside OS X and iOS are not supported at this time.
iCloud is available for Windows, yes.
The downloads are available from https://developer.apple.com/icloud/downloads/
I haven't found documentation explicit for Windows yet, though. Your best bet is to do a scan on Apple's "private" dev forums" and do a keyword search on "iCloud" & "Windows" and you'll see plenty of chatter asking the same question as you.
A blog post on icloudev.com claims that during the WWDC 2011 iCloud session, it was mentioned that limited iCloud APIs (apparently not including key-value data) would be available for Windows later in 2011.
I’m not sure if this stuff is covered by NDA — I’m not currently signed up to the developer program myself.