USB and Firewire sound cards are great, and I use them for multitrack recording and other pro-audio situations. But for everyday stuff (listening to music, composing with GPO, etc.) do you really want to have to lug around a separate card, cables, and sometimes power supply, and have to hook it up every time you use your computer?
I have an Echo Indigo I/O PCMCIA card, which stays in my laptop permanently, sticks out just about the width of my thumb, and is perfect for most of my audio needs. I use it with GPO with no problem. If I need to record multiple tracks, I use an external card, but for most stereo playback and recording purposes, the Echo is perfect and doesn't require any lugging, cables, or extra thought.
If you don't do any audio recording, and are just looking for playback of GPO and other stuff, the original Echo Indigo (not I/O) is probably fine, and cheaper.
I've been looking at Edirol USB for the super fantastic laptop I've got on order...but the benefits you've listed really caught my eye! I definitely don't want to lug nothin' around if I don't have to and the idea of a card semi-permenantly taking residence (mostly) inside the hardware is also too good to pass up.
I have had good success with the Echo Indigo. I've use it with GPO at many trade shows and hardly a problem. Soundblaster just recently came out with a PC Card but I have not used it yet (I''ll be testing one soon).