I've given this a lot of thought and research, as I am working with a group to estabish online poker guidelines in WA State. So far, I have found nothing that would preclude playing on a site that had a player pool that included other states or countries. While the Fed has jurisdiction to regulate interstate commerce, it doesn not seem to me that the lack of such regulation equals prohibition.
For instance, the initiative we are working on requires an in-state presence. So presumably one could start a poker room as a "skin" on for instance the Merge Network, and would be able to offer play against the network's entire player pool, while protecting WA players' interests and insulating them from future issues with the law.
pokerstars does have closed pools in at least one country, and if it penciled out for them, could certainly do the same for a specific state, if it deemed the safest way to operate.
If the Barton Bill is passed, this will all change. There is language in that bill that will pretty much require that only large
casinos, and/or slot machine operators, will be able to be licensed to offer online poker. That a cardroom would have to be licensed for "at least 150 tables" excludes WA state, which has a maximum of 15 tables in poker establishments. Some of the language in that bill serves the large casino interests but puts a start up company, and existing
poker sites, behind the 8-ball.