There are lots of places other than enTourage to buy books, and I believe if enTourage closed shop, books would still authorize with Adobe (I hope - if the ebooks you buy from a store have to authorize against that store's servers, then we've got a problem - not just enTourage, but ebooks from any store).

However, one thing that would stop working is the Registration Wizard on the eDGes, which means we could continue to use the eDGe we now own, but if we buy or sell them, the new owner won't be able to register them, making the eReader non-functional for DRM-protected ebooks. The LCD reader apps would still work, of course. This would cut the current resale value of eDGes in half, if not by more... (and if there's no Froyo, then they'll have next to no resale value)