Recently I proposed
to add digital signatures to bitcoin: URIs, in order to proof the identity of the payee (see http://ecdsa.org/bitcoin_URIs.html
Here is another feature I would like to see added to bitcoin: URIs: expiration dates.
Here is an example: I recently bought a gift order at spendbitcoins.com. The merchant webpage displayed a bitcoin address and asked me to initiate payment within 15 minutes. I guess the 'within 15 minutes' constraint is in order to allow the merchant to delete the token if the customer does not send the money. I think this type of constraint should make it into bitcoin: URI specs. Example:
The expiration date could be a block number or a date.
the expected behaviour is that a Bitcoin client should not send the coins if the URI is expired.