And a related question is: Which packages are searchable by Hoogle?
The best answer to that is "some". I intentionally excluded OpenGL and other graphics ones because they have a large interface and yet are not used by most people using Haskell. I have recently patched Haddock so it will directly generate Hoogle information, and am in the process of modifying hoogle so that you can pick which libraries or applications to search. Hopefully once this is all done, the standard web hoogle interface will allow searching OpenGL libraries, if the user selects that as an option - and also allow searching other libaries/applications such as GHC/Yhc/Darcs/Gtk2Hs as selected by the user. Surprisingly, a large proportion of GHC gives errors after being run over with Haddock, particularly the type directory - if someone fixed this it would be handy :) Thanks Neil