Developers working on the iDroid project have managed to get the Linux kernel running on an iPad and iPod Touch 4G. A full distro is a long ways away, but this is an impressive step towards getting the flightless OS installed on absolutely everything that has a current flowing through it. Due to the similarity in hardware, current progress applies to all A4 iDevices.
The iOS port of Android is continuing to make progress, and is actually pretty usable in its current state. If you’d like to help develop or test it out for yourself, you can check out both projects at Github.