With regards to the bitcoin client, I get your point, I never thought of that, It would have to be handled by bitcoin devs themselves
That still relies on trusting someone which goes against the spirit of bitcoin because the protocol is trustless.
Most people know the first/last few letters of their bitcoin address and would surely notice any changes.
I'm not sure who your target user is but while people might know their own bitcoin address, when they send their user.mybtcaddr.com address to someone, the receiver has no idea if the address was tamperered with or changed even temporarily and this could be quite an expensive mistake.
Programs like vanitygen can generate similar looking addresses.
The following are generated that start with 1mfr (from your username as an example) and end with 3
Address: 1mrfyA3HstyMjGrdVRQQCsGoSR8VsExy3
Address: 1mrfESSa9PpSpgsdiszN1T3ABG1uqrEd3
Address: 1mrfJJBJa9Cq6JNMNE9VHeac4RexzM7t3
Address: 1mrfokfnEi8fMWH4oa5kzz28aDYZFCLp3
It would be good to be able to come up with an anti-tampering idea though, if anyone can think of anything.
I think it will be really difficult to achieve that in way that makes it really simple for the average person but I could be wrong.