There are a few of those types of installations today, mostly in medical facilities I believe. Guest access is somewhat of a new idea for most military installations, but people are starting to wake up to that idea today with the proliferation of mobile devices that can't easily be connected to NIPRnet but which do have a legitimate purpose in the facility. This sort of guest access dumps your connection out to a commercial ISP though - not to an internal network.
I don't think guest access on .mil is something we've seen so far, although I think there is probably a use case for it. In short, "How do I let military personnel travel between bases/camps/facilities and give them access to the local network?" With WLAN, the big problem here is a requirement for CAC authentication, WPA2 encryption with EAP-TLS, etc. You really need devices that are pre-configured for a particular network to do this - expecting a person traveling from his home base to another military facility to configure his system for WPA2 by himself is probably not practical. The USAF solved that by having the same SSID appear on every base, and as long as you have a CAC you can get authenticated. That doesn't really help non-USAF personnel though. So seems like there is an opportunity there, probably for someone like DISA who has a DoD-wide mission.