I’ve created a patch that disables receiving of IP transactions by default. They can be enabled with the “-allowiptransactions” option or “allowiptransactions=true” in the bitcoin.conf file.
Here’s the patch: http://github.com/aceat64/bitcoin-patchwork/blob/svn/patches/disable_ip_transactions.patch
I’ve already incorporated the patch into my custom client: http://github.com/aceat64/bitcoin-patchwork