As I recall, man pages are actually troff formatted text. Some people write the troff by hand, others write the pages in another format (e.g. XML) and then convert to troff and other formats. If you want an actual man page editor, there is a program called ManEdit that provides a GTK+-based GUI for writing man pages. I've never really done much with it, but it's the only such program I know of.
And just FYI, if you don't like viewing man pages from the command prompt, KDE offers a man:/ KIOslave that lets you view man pages as web pages in Konqueror. You can try it out by entering man:/programname in the Konqueror address bar.