Dear Allan,
I compiled the InputManager from svn and installed it similar to what I found here on this list's archive (i.e. put both the .bundle and Info into ~/Library/InputManagers/Edit in TextMate --- this works! Great, now I can write my emails in TextMate (which is sooooo much better than Mail.app) using Markdown etc.
Also the problem with the PropertyList bundle was odd. I deleted the bundle from Disk and re-imported it from the svsn and now it is available (again?).
And as a side note and feature request: Would it not be really great if TextMate would take over in ALL Cocoa Apps? ;) Not just as an InputManager, but as default editing view ;) Ahh, just dreaming again.
Daniel