There was a suggestion for being able to "add" fees to an existing tx to ensure it does get processed but I am not sure whether this idea has been looked into in detail yet.
There is the feature where the recipient can then spend the yet unconfirmed funds and pay a fee that will incent the miner to include both transactions to earn the fee.
As far as I can, see the ability to modify an unconfirmed transactions needs to be implemented at some point, since mining fees can't be properly determined unless miners are willing to replace an unconfirmed transaction with a new one that includes a higher fee.
If the punishment for not including a high enough fee is literally locking the coins in that transaction, then there is no way to determine - for a sender - how low a fee he can include.
This will, of course, make accepting unconfirmed transactions more risky, but I think accepting these is bad practice anyway, and the negative consequences of not being able to determine the lowest possible fee is worse than being somewhat able to rely on unconfirmed transactions - which were never reliable in the first place.
At some point I think the Satoshi client will need to implement this, since when mining competition increases, miners who see two transactions spending the same output will have financial incentive to include the one with the highest fees, regardless of if this transaction was sent out later. The miners trying to make unconfirmed transactions more reliable, by not including the same transaction with a higher fee if it is seen later, will lose revenue compared to the other miners, and will have a harder time staying in business.
