Rather than a new console, this update is for a new accessory. The Gameboy player, as it suggests, allows you to play Gameboy/Color/Advance games on the Gamecube.
I picked this up from eBay for a reasonable price as it was the unit only without the required disc. But since my Gamecube is modded I can run the GBI software instead which is a community version of the player software and is apparently better in every way anyway.
The unit sits underneath the Gamecube and utilises the ports on the bottom of the Gamecube. A very nice subtle upgrade.
Now when the batteries run out on my Gameboy Advance, I have the option of switching to the big screen and continuing my game from where I left off. The picture quality is also pretty good coming through the OSSC and into the plasma TV in the bedroom.