<p>It seems like you could use a mashup of Relector and a Debugger to be able to debug any .NET app WITHOUT having the source code at all.  Is this possible?  Has anyone seen this before?</p>

I tried this a long time ago without success. Reflector has improved alot since then so I imagine it may be possible today.
It's actually kind of scary if you think about it. Someone could decompile your app and have the full code, then modify it and distribute their own version of it. All without being open source. But then again that's why "they" created obfuscators.