You guys are rather behind the times.
For an easier time exploiting the PSO/BBA hack uploading ISOs to your GCN, look for the Pheonix Tools. It's been a few months since I've played Mario Kart: Double Dash using the method (before you accuse me of not supporting Nintendo, I'm a proud Nintendo fan-boy owning 46 GameCube titles, 4 Platinum WaveBirds, the GCN BBA, GameBoy Player, and some third-party accessories).
Anyways, I was just googling looking for a better GCN ISO Ripper, since I would find an easier time ripping GCN games myself, rather than using BitTorrent. The Pheonix Tools' ripper is the only thing in the set of tools they provide that doesn't work properly.
WarpPipe Group is a bunch of guys working on their software that tricks Mario Kart: Double Dash, 1080 Avalanche, and Kirby: Air Dash (I think is the name) into thinking that the PC is another GameCube. With various ways of creating the set-up, via two NICs in one PC, or using a Router, etc., online play has been extended beyond just Phantasy Star Online. And so with a regular BroadBand internet connection you can play with others around the world for the first time on GameCube. Too bad Nintendo didn't release a program like this.
Now for people interested in Emulation, the Dolphin GCN Emulater is coming along RATHER nicely. And Homebrew is...well it's not an exciting time for that just yet, at least not for end-users.
The GameCube Broadband Adaptor is only a 10MBPS unit, and the port from the adaptor into the GameCube is about as limiting. It is unlikely that load times via the PSO/BBA set-up will ever be able to surpass the GCN's own Optical Drive.
And it is true that the GCN Discs are propietary. ISOs can not be burned to run on GameCubes. HOWEVER...if you want to spring the $400-$600 for the Panasonic version of the GameCube (named the Panasonic Q, you can buy it from lik-sang.com) then you can burn GameCube ISOs on regular size industry standard DVD discs, and play it on a completely unmodified Panasonic Q. It has been tested and confirmed to work.
Panasonic Q is a Japanese system and can only be bought via import. However, lik-sang.com offers a pretty English Face-Plate for the Q's bundled Remote Control. The Q is designed to be a home entertainment center, capable of playing DVD's, Audio CD's, and of course regular GCN games. It has a language select available in the Q so you can choose between various languages for your GCN BIOS load-screen (the pretty rounded cube).
Before you get excited and think of the Q as something that must have just been released, it's actually been around since about the time the GameCube itself was released. So like I said, you guys are really behind the times, aren't you?