Essentially, my understanding is that all the shares are held by a common trusted corporation, the Depository Trust & Clearing Corporation (DTCC), or something

Essentially, my understanding is that all the shares are held by a common trusted corporation, the Depository Trust & Clearing Corporation (DTCC), or something like it. In this way, transfers in ownership are as fast as updates to their ledger about who owns what. I suspect it's only a matter of time before something like the DTCC is established for Bitcoin. Yes, from one perspective it would defeat the "trust-less" part of the system, but from another perspective it would enable instantaneous trades both within exchanges and between them. All that's necessary for this scheme to work is for the exchanges and other market participants to trust that the depository company will deliver on any assets it claims to owe them.