Packages that depend on gtk2hs-buildtools fail to compile:

$ bauerbill -S --aur hledger-full
[...]
--> Building hledger-full...
==> Making package: hledger-full 0.12.1-2 (Thu Oct 21 14:28:19 CEST 2010)
==> Checking Runtime Dependencies...
==> Checking Buildtime Dependencies...
==> Retrieving Sources...
  -> Found hledger-0.12.1.tar.gz
==> Validating source files with md5sums...
    hledger-0.12.1.tar.gz ... Passed
==> Extracting Sources...
  -> Extracting hledger-0.12.1.tar.gz with bsdtar
==> Entering fakeroot environment...
==> Starting build()...
Configuring hledger-0.12.1...
Setup: At least the following dependencies are missing:
gtk2hs-buildtools -any
    Aborting...
ERROR: makepkg exited with an error (512)

The package *is* installed, however, it's just that GHC doesn't know that:

$ pacman -Qi gtk2hs-buildtools
Name           : gtk2hs-buildtools
Version        : 0.11.2-1
URL            : http://hackage.haskell.org/package/gtk2hs-buildtools
Licenses       : GPL2
Groups         : None
Provides       : None
Depends On     : gmp
Optional Deps  : None
Required By    : haskell-cairo  haskell-gio  haskell-glib  haskell-gtk  haskell-pango
Conflicts With : None
Replaces       : None
Installed Size : 3616.00 K
Packager       : Unknown Packager
Architecture   : i686
Build Date     : Thu 21 Oct 2010 12:09:07 PM CEST
Install Date   : Thu 21 Oct 2010 12:09:15 PM CEST
Install Reason : Installed as a dependency for another package
Install Script : No
Description    : Tools to build the Gtk2Hs suite of User Interface libraries.