If you'd like to jump on the bandwagon, backported code for BIP 16 is available at:
https://github.com/gavinandresen/bitcoin-git/
... in the "p2sh_backport" and "p2sh_backport_vinced" branches.
Backports are available for all releases from bitcoin version 0.3.19 forward; for example if you're running code forked from the 0.3.24 release you would:
Code:
git checkout -b bip16 # Create a branch for the changes, just in case
git fetch --tags git://github.com/gavinandresen/bitcoin-git.git p2sh_backport
git tag -l 'bip16*' # List the backports available
git merge bip16_v0.3.24_1
The "vinced_mergedmine" tags are for if you are using Vince's 'getauxwork' patch/branch for merged mining (based on bitcoin version 0.3.24).
If you're running latest&greatest or are willing to upgrade to the latest&greatest, BIP 16 support is already in https://github.com/bitcoin/bitcoin/
Finally, if you do decide to support BIP 16, upgrade your code, and start mining with it, let me know and I'll be happy to thank you publicly in my signature (offer good until I run into the 300-characters-in-the-signature forum limit).