This morning I was unable to do post-review from my source tree. I got the error stating:
"abort: couldn't find mercurial libraries in [/usr/platlib/Library/Python/2.6/site-packages ...

So, I tried updating Python through brew:
$ brew update python

This lead to a permission error:
/usr/local/Library/Homebrew/global.rb:43: warning: Insecure world writable dir /some/dir in PATH, mode 040777

Ok, so I changed permission to 755 on the directory.

Afterwards the brew update python gave me this:
From https://github.com/mxcl/homebrew
7ec5e5b..9b7c61b master -> origin/master
error: The following untracked working tree files would be overwritten by merge:
Library/Formula/argp-standalone.rb
Library/Formula/cocot.rb
Please move or remove them before you can merge.

I removed these files:
$ cd $(brew --prefix)
$ rm cocot.rb

However, removing these files didn't help with the brew update. Instead I had to manually update brew through git:
$ cd $(brew --prefix)
$ git fetch origin
$ git reset --hard origin/master
$ brew update
Already up-to-date.

Also, python was up-to-date:
$ brew update python
Already up-to-date.

Well, then, did post-review work? No, I had to download Mercurial and install it: http://mercurial.selenic.com/downloads/

That got me finished:
$ post-review
Review request #NN posted

Hope this helps anybody.