Parasite Eve was released side by side last night with Dissidia Duodecim Prologus on the Playstation Network, it's playable on both the PSP and PS3. It's currently $5.99, to buy it either go on the Playstation Store via your PSP or PS3 or you can access it by downloading Media Go off the Playstation Network website and plugging in your game system that way.
Also, if you were planning on buying Dissidia Duodecim 012 to swap Aya and Lightning's outfit, I suggest picking up Duodecim Prologus as well. It will give you Final Fantasy VII's Aerith as an exclusive Assist and gives a couple exclusive items. Also with a $2.99 asking price, it's a rather good deal.
No word on when Parasite Eve will be on the European Playstation Network, but Duodecim Prologus will be there some time in between Friday and Tuesday. If they'll joint release it like they did in United States then it'll be released then too.
Source: Myself, haha, I bought both Prologus and Parasite Eve for my PSP.