Looks like GPL is an ever expanding world. Through slashdot, I learned that there is GPL IDE for C# called SharpDevelop. Given that there is nothing wrong, apparently, with C# as a language, maybe I’ll need this one day. It is also being ported to Mono so it should eventually run under Linux too.
There is already a fork of SharpDevelop that allows it to run on top of mono, aptly named MonoDevelop. I have not tried it myself, but it supposedly works fairly well.