Although not as simple as turning an agent off for each IP address or Receive Connector, Exchange 2007's new transport permissions model allows you to do this just as easily.
The ms-Exch-Bypass-Anti-Spam permission is what allows a sender to bypass antispam agents. By default, mail from authenticated senders is not filtered.
To allow unauthenticated/anonymous senders to bypass antispam filters on a particular Receive Connector, use the following command:
Get-ReceiveConnector "My Receive Connector" | Add-ADPermission -User "NT Authority\Anonymous Logon" -AccessRights ExtendedRight -ExtendedRights ms-exch-bypass-anti-spam