I needed to apply the follow patch to packedstring to install it with ghc-6.12.1 and cabal-install-0.8.0 on my machine:

diff -u packedstring-0.1.0.1/packedstring.cabal\~ packedstring-0.1.0.1/packedstring.cabal
--- packedstring-0.1.0.1/packedstring.cabal~ 2010-01-17 16:23:32.000000000 +1000
+++ packedstring-0.1.0.1/packedstring.cabal 2010-01-17 19:18:15.000000000 +1000
@@ -9,7 +9,7 @@
    (Deprecated) Packed Strings.
build-type: Simple
Library
-  build-depends: base, array
+  build-depends: base >=4, array
  exposed-modules:
      Data.PackedString
  extensions: CPP

(sorry gmail is probably going to mess I the patch header)

Jens