Wow, it's finally happened. The libtiff exploit that allowed users to downgrade their PSPs has now been used to launch a game application. Although a bit primitive, this piece of homebrew from noobz can be installed and launched very easily. Simply download this zip file and extract it into your PSP/PHOTO folder. In the PSP, browse to PHOTO and you'll be able to execute the game. Easy as pie, huh? With the release of an SDK, we're undoubtedly going to see much more soon.
Hope you didn't upgrade to 2.81.
[Thanks Justin B!]