Michael Mace hit upon this over 2 years ago:
See "Toll free" Applications
in the text, I'll wait.
The implications are that it needs to be implemented the same on all carriers. A nigh-impossibly tall order.
The most impressive part is that toll free apps would mean everyone's phone could conceivably use them -- a phone with no data plan, or maybe a prepaid data plan -- could use as many toll free apps as they wanted with no financial impact on their account since the app provider/publisher/whatever is footing the bill.
That means if you have a phone you aren't left out of the mobile internet, and you are a consumer on the mobile internet, enabling all sorts of crazy new commerce.
Unless of course the carriers remain deregulated to where they can continue to obliterate all market-creating and market-expanding innovations like this would be.