This feature is not going to work on the live Bitcoin network until the OP_RETURN length is increased from 40 to 80 bytes, which will happen in Bitcoin Core v0.11, currently scheduled for release on July 1 2015. Until then, we'll use the Bitcoin testnet, but username registrations will be discarded when the move to the live network happens.
It's past July 2015 now; looks like it should be done? https://github.com/bitcoin/bitcoin/pull/5286