A couple of years ago when I first read the bitcoin whitepaper and was very impressed and excited (how can you read that and *not* be impressed and excited?) I proposed a slight change to the bitcoin protocol to add support for messages querying for "elided" blocks, i.e. a means for querying a peer for blocks only relevant to a given transaction (the history of all addresses relevant to and leading up to that transaction). The elided blocks would just be the full details necessary to allow a client to validate a transaction, with the Merkel tree being used to elide most of the data.
With a feature like this, a client would not need to download the entire block chain, they'd just need a) enough of the block chain to be able to validate elided blocks which wouldn't be much, and b) a peer willing to answer elided block queries for them. Since answering an elided block query takes real work (you have to have the entire block chain indexed in such a way as to make answering such a query efficient, which means storing alot of data, with proper indexes, and proper software, and connecting to the bitcoin transaction firehose, with the cost associated with that), I included a mechanism that would facilitate allowing the peer to 'charge' you for this service, using the same pseudo-anonymity of regular bitcoin.
The idea being that a client should not have to trust a third party to handle their transactions for them, which is the only feasable way to do bitcoin transactions now unless you want to download 15 GB - not really feasable in most circumstances - and connect to the firehose - also not feasable in most circumstances, and would be much less so were bitcoin to actually become used with any real transaction volume on a global scale. For a small fee (probably cents per transaction I would guess) you could use a system that didn't require you to trust any peers (unlike the current "we'll hold your wallet for you and do your transactions on your behalf" services that seem to have proliferated to make bitcoin actually feasable for clients).
I started to write it, but then gave up on it because I lost interest. All I got out of it was a couple of bitcoins I bought for fun, that made me about 2 grand (on paper of course, I never sold them and I don't really think it's possible to actually liquidate bitcoins into real money without serious work and headache).