docs on contrib modules that can't pg_upgrade?

Howdy folks,

Was wondering if there are any docs on which contrib modules don't work with 
pg_upgrade? I seem to remember discussion on this during the 9.0 cycle, but 
couldn't find it in the mail archive, and don't see anything in the wiki.  What 
brings this up is I'm currently working on an 8.3 upgrade and it has 
pg_freespacemap which breaks things; I think easy enough to work-around in 
this case, but I am sure for other contribs, or for folks with a lot of 
machinery built on top of a contrib, that won't always be the case. If 
something like this doesn't exist, I'll start a wiki page on it, but thought 
I'd ask first.

-- 
Robert Treat
Play: http://www.xzilla.net
Work: http://omniti.com/is/hiring