I'm not sure if this idea has come up before, but I haven't seen anyone discussing it so I'll post it anyway.
One of the problems with bitcoin, probably one that will get worse over time, is the time it takes to validate payments. But with the rising use of online wallets like blockchain.info, I think there could be a way around this. Lets say that a majority of users have some of their coins hosted with one of a handful major wallet providers. What if these providers had a publicly available interface for checking if a certain transaction comes from one of their wallets (only with the wallet owners approval, of course).
In this situation merchants receiving payments could quickly check with the wallet providers to see if any of them can verify the payment, and if so, confidently (to the degree they trust the different wallet providers) proceed with the transaction without waiting for any confirmations in the block chain.
Such a scheme would give the option of quick transactions in many situations, while leaving the blockchain as a fallback in situations where the user is not using a online wallet (or don't want it known that they are), or where the merchant does not trust the wallet operator, or maybe think the amount transferred is big enough to justify waiting for proper confirmation. The merchants, or even third parties, may verify the information given by the wallet providers with the eventual block chain confirmations, thus giving strong incentives for wallet providers to be honest.
What do you think of this idea? Is it something that has been discussed and shot down before? I can't really see any big problems as this solution is totally voluntary for all parties and doesn't require any changes to the bitcoin protocol itself.
What are you describing is already done with green addresses:
http://en.bitcoin.it/wiki/Green_address
