Here's a simple way to move your swap partition under Leopard that appears to work for both Intel and PowerPC based Macs. Edit the file com.apple.dynamic_pager.plist, located in /System/Library/LaunchDaemons. Replace the /var/vm path with the full path to your swap partition. In my case, the entire plist ends up looking like this:
Save and reboot, then check your new swap location is being used with this command:ps -wax | grep dynamic_pager -m1
Finally, you probably want to clear out any old swapfiles from /var/vm:
sudo rm -rf /var/vm/swapfile*
As mentioned this works on PowerPC and Intel Macs, and it also works after reboot or shutdown/restart. However, if your swap partition can't be found, there is no fall back -- Leopard will start up without swap, and may get unstable if you continue to use it in that state. There's a more verbose version of this tip in this forum thread, with some extra discussion regarding the sizing of the swap files.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071121070305908