Excellent explanation creighto, thank you.
So theoretically, as bitcoin expands, we'll see fewer and fewer people solo mining, more and more people pool mining, and more and more people connecting to the pool servers as nodes to verify blocks/transactions for their thin clients. I can see this working...
Personally, I predict that trusted peer services will spring up, eventually, with the promise that transactions sent from paying users' clients will continue to be processed by the "bitcoin bank's" own pool and/or datacenter for free, even if it might take some time. This might also become a privilage of pool contribution.
How difficult would it be to implement a new "thin" client? Or does one already exist?
I'm not a programmer, but my understanding is that it's already part of the protocol, but isn't yet included in the standard client for several reasons. Not the least of which is the lack of urgency as compared to other concerns, such as encryption of the wallet.dat file.