As far as I can tell, the IP address is not configurable, and there's no option except to run on 127.0.0.1.
In order to test (rails web site) on multiple devices, I always run as either a specific LAN IP others can reach, or I use -b 0.0.0.0 to use my computer's current IP.
Being able to define an IP would be much more useful.
(Obviously I can create my own Task, but just thought this might be a useful observation.)