In my opinion, an ideal solution for the coworking space would be to use m0n0wall (http://m0n0.ch/wall/) or pfSense (http://www.pfsense.com/) to configure a captive portal wifi hotspot with either username/password protection or just a web page that shows the user the terms of service and prompts them to click "I agree" before continuing.

This same device would also have firewall and rate limiting rules that would block certain things like outbound TCP port 25 (block traffic that is likely to be spam), place certain things like P2P traffic at a lower priority than web browsing/email/etc, and provide a dedicated high-priority network interface to any existing VOIP ATAs you might have. That way, you can provide wifi for free to anyone within range, and yet do so in a safe and secure manner.

This would require some hardware (I'm especially fond of boxes from http://www.Soekris.com/ and http://www.Netgate.com/), but it's likely that it can be achieved with a spare Pentium-II PC and a commodity wifi access point.

I have a small but growing page about such efforts here: http://cernio.com/community-wireless-networks/

In addition, if one wants to grow a wireless inexpensively with minimal configuration, check out Open-Mesh at http://www.open-mesh.com.  We're using it as part of a state-wide community wireless initiative at the Virginia Wireless Company (http://www.vawifico.org/)  Average cost to deploy a mesh node is about $40.  One or two mesh nodes could easily support a coworking space.

-Kory Mohr (kory @ vawifico.org)