Which IDE if any, are people using to develop Ironruby in?
Visual Studio?
According to the IronRuby website Visual Studio C# Express can be used (and in turn, any commercial version of Visual Studio 2005+ I'll assume).
From the IronyRuby.net home page:
Today, you must check the source code out of the IronRuby Subversion repository on Rubyforge. You will need a Subversion client: we recommend TortoiseSVN. To build the sources from the command line, you must also have Ruby installed on your computer already: we recommend the Ruby one-click installer. You can also build the sources using Visual Studio; if you don't already own a copy, you can download a free copy of Visual C# Express 2008.
Ruby in Steel from Sapphire Steel is build on the Visual Studio Shell (integrated mode) that will merge with Visual Studio 2008 if you already have it installed or simply be a standalone installation if you don't have Visual Studio already installed.
Also, no use to you, but Microsoft are going to be releasing IronRuby Studio (and IronPython Studio) at some point in the future. I couldn't find much about these on the web though - they were mentioned by a speaker at Teched Europe a few weeks ago.
You might interested in IronEditor. which similar to SciTE. get it at http://www.codeplex.com/IronEditor
I use Vim as my IDE with some custom settings that I blogged about some time ago.
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.