If you're interested in having a Mac OS X version Rich Notes I'd like to hear from you. Let me know if it's something you'd be willing to pay for, and how much. Click the Submit a request button at the bottom of this page or use the Contact Support button found under Info in the iOS app.
There are no plans for a Windows app, as it would not be able to sync notes with Rich Notes iOS. Only Mac OS X apps sold via the Mac App Store are able to do iCloud sync.