After some additional testing and fudging around here is what I identified:

The "forum/attachments" folder which the forum defaults to uploading attachments to didn't exist in our portal.  I thought the forum would automatically generate the folder the attachments were going to but it didn't.  I created the folder (initially logged in as Host, but had the permissions set to allow admins and a particular user security role to have "write" access)... logged in as an admin and it told me I didn't have access to the folder.  Weird.  I logged back in as host, nuked the folder, logged in as admin, created the folder and gave the appropriate rights again.  Admin had access to the folder now and it lets me attach the files through the Forum thread attachment uploader.

What I see then is that the framework is definitely having issues on the File management side of things.  On that note and yet a completely separate train of thought the Host -> File Manager doesn't work at all... it throws an error in the get_Permissions section of the code.  Nice... a Host user doesn't have access to host-only files.

I'm guessing the issue definitely lies in 5.1.2 and *not* the Forum module.  That's comforting.

Matthias, thanks for trying to help me track down the issue and figure out what was going on.