...
So of course we have SPV, which is likely going to be released as part of the official client in the next release.
But doesn't that leave us with an ecosystem made of 'supernodes' while everyone else chugs along with their SPV client, centralising the control of the block chain into the hands of a few?
...
It will either work or it won't, where 'work' is probably a little bit subjective.
My current best idea for mitigating against potential failures is an idea that hit me yesterday and is referenced in my sig block. It is, or course, a work in progress.