I just installed the generic 7.0.2 binary on Ubuntu  on both 32 bit and 64 bit machines.  Then I rm'd   .ghc and .cabal  then fetched the current haskell platform. Built and installed. 

After cabal installing all of the dependencies cabal said were required for yesod,  I run yesod.. the script creates the project.

When executing runhaskell devel-server.hs  I get this message

GHCi runtime linker: fatal error: I found a duplicate definition for symbol
   ghczm7zi0zi2_AsmCodeGen_zdfMonadCmmOptM2_closure
whilst processing object file
   /usr/local/lib/ghc-7.0.2/ghc-7.0.2/libHSghc-7.0.2.a
This could be caused by:
   * Loading two different object files which export the same symbol
   * Specifying the same object file twice on the GHCi command line
   * An incorrect `package.conf' entry, causing some object to be
     loaded twice.
GHCi cannot safely continue in this situation.  Exiting now.  Sorry.


it's the same on both 32 and 64 bit. 

Any ideas?   I am going to try to build 7.0.2 from source to see if that fixes it.