Above you'll see an emulator running Sonic and Knuckles on the PlayStation Vita. Woah. That was quick! But, before you get too excited, know that this is actually going through the Vita's PSP emulator, so its emulation inside emulation. The creator of this video, Wololo, used the Picodrive emulator and the Half Byte Loader, but "given the secret nature of the exploit and some of the loading process" he's just showing the gameplay (and the crappy sound) for now.
Here's another reason why you shouldn't get too excited. Wololo points out that Sony has security in place that will make this exploit moot as soon as it is made available. He says that this should only be considered a "ray of hope" right now, and that he's only in the research stage so far. Today Wololo has the PSP port of Doom running on the Vita.
This is neat for sure. I have always had the downloadable version of the Sega Genesis Collection on my PSP, but it's missing some of my favorites. Somewhere a Sony boss is sighing.