Well, maybe not the ultimate (I'm still waiting for a pocket Gamecube - or better). But this looks pretty damn awesome. Basically it's the GBA except somewhat more powerful, slightly better screen resolution (plus the screen doesn't suck), it can play MP3s, and it can do network/internet gaming. Oh, and it's open source to encourage third-party development. Its website is here; unfortunately, it's in Korean, but you can still look at screenshots and whatnot. Also check out here for a review (in English) of the system, and here for information on development. I'm sure there are more sites too.

Best of all - if you want one, Gord can hook you up with one. Check the most recent news item (Feb 12). I can tell you right now that I am going to try to get one, and maybe even a game or two (despite them being in Korean), providing I can afford it.

[EDIT] Ehh... whoops. As it turns out, there is an English site. http://www.gp32.com/English/main.asp