It's nice that the Ion platform includes an onboard 9400m graphics chip, HDMI capability, etc. but I doubt this will be any different for MAME than what's been on the market for the last year or so(see
this motherboard for $80). MAME performance is based on processor speed more than anything, and the onboard GMA950 of the board I just posted is plenty sufficient for MAME.
As far as what RayB posted, I'm not sure how the front side bus speed of this motherboard is a problem. Yes, the Atom is only a 1.6GHz chip, and a slow one at that, but I hardly see it being lucky to play any games newer than 1990. Mortal Kombat 3 should run on this board, and that's 1995. NBA Jam TE will run, that's 1993. I don't own a board with the Atom 330, but I do own a mini itx board with the atom 230(the 230 is a single core atom. the 330 is the dual core), and even that board is at least comparable with a 2.4GHz Pentium 4. So when you have a board with the 330(theoretically twice as fast as the 230, if the app takes advantage of it) it should be decent for MAME performance. Probably won't be rocking out Killer Instinct with it, but it would be a great replacement for those with aging Athlon XP systems and Pentium 4's. Draws a lot less power too, so you'll save on your electric bill, and your cab won't get as hot.
So, in short, the ion platform is cool, but save yourself ~$100 and just go for that motheboard I posted. The MAME performance difference will be negligible. If you'll be using it for any kind of PC games, though, or need HDMI or 6 channel audio, go for the Ion.