MMS in the appstore
There is now an app in the appstore called MMS Photo Messaging
It allows you to receive for free but it currently costs about 35p/msg which is payed with credits bought through the app with paypal. They state that in the future they hope to lower the cost and even maybe get it to come out of your allowance.
Currently it looks like it only works if you are on o2, but its a step in the right direction for those who dont want to jailbreak at the moment.
This app only works for O2 Customers.