The Register has compiled rumours from a couple of different sources that suggest Apple is planning a sub-$100 iPhone with a 4GB capacity.

The sources alone might not hold much water, but together they seem quite convincing.

Engadget has apparently seen a PCS Type Review Certification Board (an industry group that vets GSM handset's compatibility) approval of four different iPhone models: 4GB, 8GB, 16GB and 32GB.

This is accompanied by The Boy Genius Report, which found a slide from Canadian telecom provider Rogers Communication labelled as "Apple iPhone (4GB)".

While it seems like something of a step backwards - especially when everyone's expecting the iPhone's memory size to increase, not decrease - there could be an explanation for this new model.

Apple is known to be interested in getting the iPhone into emerging markets, but the current costs don't translate well for countries such as China, India and in South America, but rumours from April spoke of an 'iPhone Junior' with a few reduced functions to cut the price right back.

This 4GB model could well be the cheaper iPhone designed for the BRIC markets, so even if the rumour proves true, it's perhaps unlikely that we'll be seeing a sub-$100 iPhone deal in territories where the device is already established.