I think it will be pools, who provide double spend protection for bitcoin billing: they will get payment in exchange on building block chain on blocks with "protected" transactions in case of double spending attempt. You make contracts with several pools to be sure you are well above 50%+, and you can accept bitcoins instantly.

And it's in the interest of such billing to publish transactions openly, so other miners could use them (or else they will be generating orphaned blocks in case of double spend attempts).