I believe there already are closed loops. Perhaps not 100% closed but almost. Many Bitcoin Internet startups have the ability to pay pretty much all of their expenses in bitcoins. Our company pays a portion of our expenses in bitcoins, and we also accept bitcoins in our online store.
I know many other companies in Finland alone that pay a significant amount of their expenses in bitcoins, and accept bitcoins for something.
SatoshiDice is, as far as I know, an example of a completely closed loop. Everything, in and out, is done with bitcoins. If there are other examples of completely closed loops, I'd like to hear about them.
Interesting. What expenses are paid in bitcoins?
I'm also wondering if operating this way will enable any efficiencies that are large enough to attract attention from competitors (besides S.Dice).