Guys thanks so much for all these pointers and ideas. Lots of food for thought here. I find for the MobileFrontend perspective, in a perfect world, all our existing browser tests would visual diff a screenshot at each individual step of our tests against the previously known screenshot and report any oddities.
Will sit down and think about this some more but thanks so much - and please don't forget this thread - post any work you do in this area here so we can keep tabs of what the options are!