Gridlee (aka MAME4iOS aka "free MAME app for iOS") is currently available. I'd get it soon. It's free.

It apparently works best on iPad 4 or iPhone 5, but even if you don’t have one of those today, it’s a good idea to download this.

I assume that when someone @ Apple comes in to work tomorrow it will be their job to remove this from the App Store (which doesn’t allow emulators).

ps - this will be nothing to most of you, but to some of you, it is very exciting.