The latest version of GHC in the portage tree is 6.10.3, but haddock-2.4.2 has an explicit dep on GHC 6.10.2. However, this package works perfectly fine with 6.10.3. Changing ~dev-lang/ghc-6.10.2 in RDEPEND to >=dev-lang/ghc-6.10.2 allows me to install the package.
Thanks for reporting this. We already had a fix in the overlay, now I've pushed it to portage too.