I have had the Membership and MarketPress plugins working happily together on a client site (www.yoga4arthritis.com) for many months.
Today I was advised that regular visitors (not logged in members) cannot access the store (http://yoga4arthritis.com/store/products/)
I have taken a look at the settings for the Membership plug-in and can see no reason why the store is coming up as protected content.
i tried adding a positive rule for all Marketpress, but that didn't work.
I set the existing free access level to inactive andf created a new one, but that didn't work either.