Microsoft Research has an experimental way to attach one Wi-Fi card to multiple networks - by creating virtual Wi-Fi cards in software on the PC.
The VirtualWiFi software, which can be downloaded here, will allow ad hoc connections to local machines which can share another Wi-Fi connection to an Internet gateway. It can make a "mini-mesh" and extend Wi-Fi to machines that are out of range of the Wi-Fi access point.
The software implements multiple virtual WLAN cards on the PC, each of which uses a time-share of the actual Wi-Fi card, and has its own IP address. It is very beta, and not yet suitable for business use as, amongst other things, it does not yet support 802.1x, or even the old insecure WEP encryption.
In future, however, it may come into its own, as the the research team is looking at a couple of useful applications. For example, a connection can be kept open in the background for diagnostics: "Client Conduit is a tool that provides a thin pipe of communication between disconnected clients and back end servers that perform wireless diagnosis and recovery," says the Virtual Wi-Fi page.
Another possibility is that VirtualWiFi might increase the capacity of wireless ad hoc networks by using multiple channels, in a technique the team calls Slotted Seeded Channel Hopping (SSCH). An 802.11b/g card would then be able to use three channels, and the wireless access point would see three separate Wi-Fi cards.
"It includes a lot of tricks and heuristics so that the periodic channel hopping doesn't add a lot of latency or cause a lot of packet drops," commented one reader of the Engadget blog. When one virtual adapter hands over to another, it tells the access point that it is going to power saving mode, so the access point will buffer incoming packets for that virtual adapter, and the overall throughput is increased.