Update: It has been pulled, you'll have to jailbreak to get an NES emulator now.
We're not sure how this one slipped through, but Nescaline, a Nintendo Entertainment System emulator is in the App Store as we speak. Yes, Apple has prohibited emulators in the past as a security measure although it did eventually approve a Commodore64 emulator. Nescaline comes with five homebrew apps but allows you to download NES.roms from the Internet by providing
