Well this was long overdue, but necessary given the impending N-Gage and the hype surrounding the PSP. Its not clear if this is only for connectivity among GBA's, or if this will extend to linking the GBA to your 'Cube, but I doubt it. (Still, the ability to use your GBA like a Wavebird would be nice, especially when FF:CC comes out next year.)
As cool as this sounds, given that I don't have anyone to play multiplayer with (well, there's my fiancee, but we only buy one cart of each game between the two of us, so we couldn't play multi without shelling out an extra $30), I can't say I'm interested in this. Combine that with the fact that I'd need to buy one for each GBA (as opposed to buying just one cable to connect two units) and its not a financially viable alternative. Still, this is something that Nintendo should have natively built into the SP, if not the original GBA, and I'm glad to see it finally being offered.
I just hope that you don't HAVE to buy the new Pokemon to get it.