Instead of building complicated authentication systems, why not employ agents and tokens in a peer-to-peer model?: Elias Efstathiou, a Ph.D. candidate at the Athens (Greece) University of Economics and Business in computer science, wrote to point to a paper he and George C. Polyzos co-authored entitled "A Peer-to-Peer Approach to Wireless LAN Roaming" (PDF download).
The two authors propose a peer-to-peer system in which autonomous agents store authentication and pricing information (although pricing can be more of a concept than actual monetary value), and use tokens to exchange access units. In this system, you don't need a monolithic back-end, because each entity has its own database, however large or small. Instead, the burden is shifted to managed token-based authentication.
It's an interesting idea, and certainly collapses complexity. However, it would have to be implemented and adopted through the same business realities that limit roaming today. Their paper presents ideas to cut through technological complexity, but you still have to have networks willing to buy into the notion of unhindered roaming to participate.