Ok, so I did some number crunching.

The Radeon Mobility 4670 gets 674% higher performance than the Intel GMA 500 (based on PowerVR 535) in Fear, which is a very shader intensive game.
http://www.notebookcheck.net/Intel-Graphics-Media-Accelerator-500-GMA-500.12614.0.html

According to Anand, the modern PowerVR chip used in the iPad 2 gets roughly 5.5x the performance of the GMA 500 chip in shader intensive tests.
http://www.anandtech.com/show/4216/apple-ipad-2-gpu-performance-explored-powervr-sgx543mp2-benchmarked/1

This means we can expect a modern PowerVr chip to 81% of the shader performance of a Mobility 4670. The M4670 is equivalent to a desktop 4650, which can manage 31 Mh/S.
https://en.bitcoin.it/wiki/Mining_hardware_comparison

We can therefore expect the PowerVR to manage 25 Mh/S at .9w. This gives us a Mh/J of 27.7, compared to 1.8 for a 5850.

This has potential.