Hi Arthur,
Glad to see we have a few more fighters here 
I too have been thinking of what we need to have to create a driver, please see my post on Imagination Technologies forum, SGX545 driver issues - Imagination Developer Forums I would imagine we need some idea of how the GPU communicates with the CPU, then the different ways in which the GPU processes different graphical components, and then finally what the GPU does with it's solutions. I have no experience with GPUs' or anything else driverwise, so this is my basic level thinking. Now from what Imagination technology has already told me, my guess is that we do need source code for this, and that Imagination technology are not desperately keen to give anyone this. So yes there are definitely IP problems, if that's what you meant about legal problems.
After reading my post and Imagination Technologies reply, some feedback would be excellent on what you think the situation is, otherwise this could easily descend into conspiracy theories 