Lightspeed Aviation's FlightLink is a free iPhone/iPad app for folks with a Zulu 2 headset that records all intercom and ATC communications as you fly. You can scroll back and replay any part of the recording, and search a library of recordings from previous days. The iPad version also has a scratchpad for writing notes.
The app will keep recording even if you switch to another app on the iPad, but it won't work with original Zulu headsets, due to a technical issue, or any non-Lightspeed headsets because the app needs a Zulu serial number to unlock. The recorder also requires the cable connection between the iPad and headset rather than Bluetooth.