The developers appear to be using an Enterprise Distribution Profile in order to get the application to install on users iPhones. This typically would allow a company to develop apps for its employees without having to release them in the App Store for distribution.
In order to install the application you will have to manually set the date and time of your device to any date before today before installing. Once the the app is installed and launched once you may set the date and time back to being set automatically.
I’ve played a few minutes of Pokemon FireRed with it and everything appears to be running smoothly.
And if your concerned about security, user t3rminus left a reassuring comment over at Touch Arcade:
I monitored the entire install process, and several minutes of gameplay with a proxy. This application doesn’t seem to be doing anything nefarious (uploading private data to remote servers, etc.), although it’s not to say it can’t or won’t in the future.
I suppose I’ll go waste a bunch of time playing Game Boy Advance games now.