In the Pokerstars FAQ they state:
"Q. Is my PokerStars account balance used for your operational expenses or is it kept in a separate account?
A. PokerStars is proud that, under special banking arrangements, an amount covering the total of all players' account balances is held in segregated accounts, not used for any operational expenses. These segregated accounts are managed by one of Europe's leading financial services groups. These arrangements ensure that PokerStars can at all times fulfill its obligations towards its players, and provides further reassurance that players' funds are always secure with PokerStars."
This is one of the areas where international regulation would help. Poker sites should be held to the highest of financial standards, similar to BnM casinos. In most U.S. jurisdictions, gaming commissions require that casinos keep enough cash on hand to pay for all of the highest payouts that 'could' hit simultaneously. The exceptions are the big mega-slot games where states or gaming manufacturers make the actual huge payouts after verification of the jackpot hit.
Lack of regulation, imo, is where playing at the smaller sites puts your cash at risk. Stars, FT, Ultimatebet, etc pose huge competition situations that require smaller sites to make huge promotional risks where they use the player deposits for funding. Now
THERE'S a crap-shoot...