As of September 2011, this command exists only in the form of a patch that must be applied to the main client.
How to get the patched source
If you are able to compile the source in Linux, you can download and compile the patched source.
First, use git to get the unpatched source:
git clone git://github.com/bitcoin/bitcoin.git
Then, change into the source subdirectory within the tree.
Then, download the patch:
curl https://github.com/bitcoin/bitcoin/pull/220.diff > 220.diff
Then apply the patch:
git apply < 220.diff
Then compile the source
make -f makefile.unix bitcoind