These apps use QR codes, but I think the way to go is NFC communication.
The benefit is instant transactions with server-client model, but on the downside you have to rely on 3rd party. I think it isn't that big downside, since you have always handed your money to 3rd party before bitcoin...
We went with QR codes for initial development because most phones still don't have NFC built in yet. I definitely agree, though. In the long term, NFC will take over. It's on the dock for future versions.
It's still early to push any cloud wallet solution for the mass market. Until any of these services build up trusted reputations, handling the transactions on the phone is the best way to go for the average user. Setting up personal servers works for nerds like us, but isn't a general enough solution.
The entire production blockchain is 14MB right now. We keep this on the SD card of the phone. Many smartphones today have 8GB SD cards or more, so 14MB is not a big deal.
I agree though, I think a thin-client or server-client model would be better long run. There just isn't a service I'd trust with it currently.
It is known that even PCs won't be able to handle the full blockchain eventually, let alone smartphones. Supernodes will be needed if bitcoin becomes popular. https://en.bitcoin.it/wiki/Scalability
edit: I see now that there is indeed a 14MB blockchain file in the source: https://github.com/barmstrong/bitcoin-android/blob/master/as.... I suppose this is not the full blockchain, but just the block headers. This is described as "Simplified Payment Verification" in Satoshi's paper.
It could very well be just block headers, to be honest I'm not sure (didn't realize the desktop version was that big). We're using the bitcoinj library and contributing back some patches along the way. Mike Hearn at Google wrote the blockstorage and deserves a ton of credit for it: http://code.google.com/p/bitcoinj/
"BitCoinJ implements the "simplified payment verification" mode of Satoshis paper. It does not store a full copy of the block chain, rather, it stores what it needs in order to verify transactions with the aid of an untrusted peer node."
There is also mtgox vouchers-app: http://forum.bitcoin.org/index.php?topic=25307.0
These apps use QR codes, but I think the way to go is NFC communication.
The benefit is instant transactions with server-client model, but on the downside you have to rely on 3rd party. I think it isn't that big downside, since you have always handed your money to 3rd party before bitcoin...