Indeed - all those look exactly like the same issue.
And the workaround: http://groups.google.com/group/happs/msg/1e7761d421b0e5eb
That doesn't fix the real issue: It causes happstack-data to not need the thing that is built wrong in syb-with-class. I believe my work-around (build syb-with-class w/o documentation) will be a more universal workaround, as at that point the syb-with-class install is no longer broken.
Here's the GHC bug report: http://hackage.haskell.org/trac/ghc/ticket/3799
Registration for that trac is broken, so I couldn't update that ticket. I see that you've added pointers to my post, thank you! Perhaps you can also add a note that the issue seems to have to do with something that cabal does differently when there is a documentation step enabled. - Mark