The folks at Go-WISP alerted me to their roll-your-own-hotspot offering: Go-WISP provides modified firmware for Linksys WRT54G gateways that allow them to connect to the Go-WISP back-end for billing and account management while presenting a login screen to hotspot users. Go-WISP charges a flat $50 per month with discounts for longer periods of prepayment to handle the back office. You set pricing and other details through a Web interface.
Sputnik has offered a similar service for a while under the label SputnikNet. But that offering is a bit more focused on managing users. There's a $50 setup fee and rates of $20 or $30 per month for a single AP based on features chosen; more APs reduce the per AP per month cost.
Firms like Airpath, NetNearU, Pronto Networks, and Surf and Sip (among others) offer directly or through resellers managed services for free hotspots and fee-based hotspots, but offerings like Go-WISP and SputnikNet are typically quite a bit simpler and require self-install.
An important note on the Linksys WRT54G: Linksys has apparently changed a number of details in newer versions of the unit. Go-WISP notes the particular releases of the WRT54G that can be flashed with their new firmware. This is under discussion all over the Wi-Fi hacker and development world, because the WRT54G has long been the device of choice between cost and access to the code for changing its firmware.