I too would actually rather run karaoke software on a Mac Laptop! I did all of my research and the only program that I found that is worth mentioning would be kjams (
www.kjams.com)... but honestly it really can hang with the likes of Compuhost for example. Software like Compuhost or Hoster needs to do a port for OSX.
The other option I was considering was running BootCamp (to run XP or Vista) on a Mac, but there were some limitations in it that wouldn't allow a video signal for S-Video... that may have been fixed in the new version of Leopard that was just released a couple days ago though.
I'm just sitting back with my (crappy) PC laptop at the moment, because it works... but if there was a rock solid OSX version of some awesome karaoke software I'd kick this thing out the window.
anyway - good luck!