A line in the sand for graphics drivers

Not really. At least for Imagination (which I suspect accounts for over 90% of mobile GPUs on the sorts of platforms we're talking about) chips, there have only ever been two: the MBX previously, and the SGX now. The MBX, which shipped in but was not used by the Nokia N800 and N810, is a direct descendant of the core used in the Sega Dreamcast - remember that? It had different integration points with every chip, but I don't remember any meaningfully different versions of the core.

There are a number of different versions of the SGX (530 in the N900, 535 shipped by some other vendors, 540 shipped by Intel IIRC), all with slightly different abilities/restrictions, but they mostly behave the same from the driver's point of view. Some of those cores have different revisions to fix hardware issues, but again from the driver's point of view, this is generally just a one or two-liner.