Yeah, that's an unreleased command. I had hoped we'd push out a new version of MacPorts with the command before Leopard was released, but we haven't. I recommend one of the following:

1) Install the latest MacPorts trunk

2) Edit the Portfile and replace the macosx_deployment_target line with

configure.env-append MACOSX_DEPLOYMENT_TARGET=10.4

build.env-append MACOSX_DEPLOYMENT_TARGET=10.4

destroot.env-append MACOSX_DEPLOYMENT_TARGET=10.4

HTH,

Kevin Ballard