Rick Stahl puts it better than me, but we have the same issue.  

http://west-wind.com/weblog/posts/1358.aspx

"The failure occurs only when trying to load an assembly into a new
AppDomain and then only if the assembly does not exist in the parent
application's AppBase or other Bin/Probing paths. Although the AppDomain is
configured to have it’s ApplicationBase in a different directory than the
EXE’s base, the remote AppDomain CreateInstance call doesn’t apparently
doesn’t use this path, instead using the current domains ApplicationBase."

Here's the *desired* architecture:

\

+-Applications
  +---- Addin1
          |--- Addin1.dll
  +---- Addin2
          |--- Addin2.dll

Thus far, I can get MainApp.exe to load the Addin1.dll and Addin2.dll
assemblies.  As they are add-ins, I'm fine with them loading into MainApp's
domain.  But Addin1.dll attempts to create a separate AppDomain.  All goes
well, including the bind (Fusion logs clean), but when I try to Unwrap, I
can't get the cast.  If I move Addin1.dll to \, I'm all set.  I'm trying to
avoid that because I'd like to constrain everything Addin1 needs to its own
directory.

Has anybody solved this type of issue?  I don't know if the whole
AssemblyResolve thing would work...  Frankly, I've learned WAY more about
AppDomains in the past week than I ever wanted to know.  Although I still
have far to go, on the whole, I'd really rather be back in Kansas with Auntie
Em.

Cheers,
M.