I've done some more tests and it seems that 300mA is the magic number, by applying 5V from a bench supply with a variable current limit. Below that it says it won't charge, above that it's fine. However even below that, it'll still take the current from the supply suggesting it is actually charging even though it says it's not.

Clearly my cheapo AA charger is crappier than I thought. I haven't actually measured what current it can supply yet. I also want to measure what the iPhone consumes when running MotionX with the display on so I can work out what to design.