Enable mirroring on iPhone 4 (over-the-air)
There's nothing that does exactly what you're asking for. However, this should be similar. I've stumbled upon it in Cydia a few times, but never bothered to try it, because AirPlay just never appealed to me. It's not explicitly AirPlay Mirroring, but it does the same thing. There's also a package called Display Mirroring and DisplayOut found in Cydia, and they enable mirroring, but it's not over-the-air (you have to use a VGA adapter). There's also another Cydia package called AirServer, and it allows your iOS device to receive media from your Mac or another iOS device through AirPlay.