i like this.
but how "baked in" to the protocol is locktime? ie it will forever be compatible with whatever official client is in use in 15 or 20 years time? a tx created with locktime today will always work in the future?
I don't think you have to worry about that
too much. Height-based nLockTime was included in the original Bitcoin implementation and time-based nLockTime was implemented in version 0.1.6.
Time-locked bitcoins are one consideration in any fork that limits transaction size -- like the sigops limit Gavin Andresen wanted to introduce. Time-locked transactions that are too large would be invalidated. I doubt such a fork would gain wide consensus, though.