I'm thinking about writing a game using DirectX, and I'm considering using IronRuby.
Yes, it is ready for production. Version 1.0 is available since April 2010. Please use it and give us feedback (link text). Regarding DirectX compatibility - I haven't tried t owrite anything but from a quick look at the managed DirextX API there seems to be no reason why it won't work. Most of the .NET APIs that are usable from C# or VB.NET are accessible from IronRuby as well. There are a few exceptions, some APIs rely on static types, but that shouldn't be the case for DirectX. We are working on enabling those as well in future versions.