Unlike correspondence chess where the state of the game is visible at all times, with poker, the state would need to be maintained on a server somewhere, and with 1 hand taking potentially a week, you'd be dependent on the server maintaining and not losing this information even during system updates. I guess the state of the game would be stored in a database. How would this be raked? As normal or at a higher fee? Is there a real market for this? The client would need to be browser based cos it's generally poker on the move?
What is the URL of the correspondence poker site?
Obviously some kind of open source peer to peer system is best but I would be happy to play on a third party host. I can't see any problem with hosts retaining information on the state of a number of games - they do this in live play and don't forget account balances etc when they do system updates so this isn't a significant issue. Rake doesn't need to be high at all as the system / server loads would be very low. Note the hands don't have to be played sequentially, rather when in a match one can play 20-50 tables at time vs one opponent at 1/50th of your usual comfortable one table buy in.
The CPoker play site saw some quite serious play last time I checked. The experience is more cerebral and quite engaging as you are changing and balancing your play on many different tables against the same player over a period of days.