Last month Apple offered an update for the Migration Assistant utility for OS X 10.6 Snow Leopard, which increased compatibility with transferring data a new Mac that ships with OS X Lion. Today Apple followed suit for OS X 10.5 Leopard by issuing an update for its Migration Assistant tool as well.
The update should be available through Software Update, but can also be downloaded from the Apple Support downloads site. The download is 4.98MB and requires OS X 10.5.8 to install, so if you are planning on migrating to a newly purchased system and plan on migrating your data then be sure to fully update your current system before applying this assistant.
The updated Migration Assistant for Snow Leopard can be found here.