Off the top of my head, this can be achieved relatively easily with a Squid proxy, but I'm assuming you want something that non-tech can roll out to a place like a coffee shop with relative ease. Modifying the firmware on a Linksys/Buffalo router could be the way to go at that point, but setting up the startup script would be the real trick. There's a lot of homebrew apps you can run like that & couple it with some script-fu you could definitely have this.
I imagine this is going to be a big proponent for the Bitcoin. If coffee shops can freely have this as passive income potential, adoption rates go up & Bitcoin exposure goes up. I am keenly interested in developing for this
I don't think most businesses would be interested in a pay portal for their customers, most already offer free internet as a side benefit to being a paying customer. I think that will eventually become the norm, much like being a paying customer entitles you to free use of the bathroom, but it's still rude to walk off of the sidewalk to crap without buying anything.
I would find this kind of thing useful at public events, say with a WiMax WAN uplink, some QoS code to keep things even among customers, and a built in Bitcoin client that all attempts to use port 8333 are redirected towards. I don't want those new BitcoinJ clients trying to repeatedly download even the blockchain headers over the WAN when they would be available locally. Put the entire system into a large lunchbox or cooler, with a couple of decent antennas neatly sticking out the top. A decent battery and/or a solar cell on the top of the cooler, and roll it all up to the highest point one can get to early in the day before the (air show, tailgate party, race, outdoor concert, fireworks show, whatever) starts, fire it up and let the crowd discover it. Some people are going to have 4G/wimax accounts anyway and simply won't care, but if the price isn't too high for a day's casual connectivity or a not-to-bad per MB rate, then those who have metered data plans are going to be intrigued. And I know that it's downright painful for some young people to have connectivity problems at any event with long periods of calm before the main event.
Sometimes, having it mounted to the top of my minivan would be even better. Another system with just the piratebox, so that both of these things can coexist in the same space, would be ideal.