Which IDE if any, are people using to develop Ironruby in?
Answer
If you are looking for an integrated Visual Studio editor, with intellisense etc, Microsoft has nothing in plan yet (according to this article with John Lam -> http://www.infoq.com/articles/state-of-ironruby). But there is an integrated editor which works pretty good - SappireSteel - at http://www.sapphiresteel.com/Ruby-In-Steel-For-IronRuby.
If you just want to edit in VS and don't care about intellisense and such, you could try and set up an external tool from within VS and call the ir.exe (http://msdn.microsoft.com/en-us/library/68c8335t%28VS.80%29.aspx)
There are numerous editors with support for ruby highlightning which you can try out as well, but there is no one (I think) with intellisense-like support for the .NET framework. Scite (http://www.scintilla.org/SciTE.html) is pretty popular, Scott Hanselman blogged about Ruby support/highlightning in Notepad2 (http://www.hanselman.com/blog/NewNotepad2WithRubySyntaxHighlighting.aspx). On Codeplex you can find a tool called IronEdit (http://www.codeplex.com/IronEditor) which I've not tried myself yet.
I ran RubyMine (http://www.jetbrains.com/ruby/index.html) for a while, which seems to be a really good Ruby IDE (costs money), but doesn't have any specific support for IronRuby. I hope the JetBrains will release something which plugs into Visual Studio eventually...
Personally I think Sapphire will come up with something really good eventually. I hope this helps.