If you haven't seen this white paper as yet, may be worth your time.  It discusses EnronOnline's place in the market now and in the future (will still be here, but much less market share).  Among other things, says companies will develop APIs to take their transaction data back to their own systems (pg. 10).  Doesn't discuss the industry clearinghouse concept for processing these transactions.

http://www.forrester.com/Research/ThankYou/PDF/0,2254,14162,00.pdf

Thanks,

Fred