We went a similar but different direction in Canada, rather than killing the phone there's a list of IMEIs for stolen phones, and all carriers will honour not allowing phones in the database on to their networks. Which this solution sounds little less onerous than re-engineering every handset OS to have this kill ability.
Also the phone doesn't actually have to be turned on to be blacklisted, how often will you send the "kill" pings out when stolen? Would a thief simply have to wait a few weeks until the heat dies down?
We have devices that register with networks when activated, isn't it far easier to wait for that event than to try and push a command to a phone that may never be turned on again?