Clever use of built-in Windows 7 networking from Connectify: The beta version of Connectify for Windows 7 uses the OS's ability to create a software access point and a virtual Wi-Fi adapter while still remaining connected to an infrastructure Wi-Fi network. This Windows 7 feature virtualizes the Wi-Fi network connection, allowing a separate client and access point function to operate as separate virtual devices using the same radio channel and same hardware. Some advantages over ad hoc networking, just like with the upcoming Wi-Fi Direct technology, is the use of WPA2 Personal (AES-CCMP flavor) for securing the connection.
The work was originally developed by Microsoft Research, and is still apparently a little hidden in Windows 7, although available. Connectify apparently lets you take one or more WAN connections (like Ethernet, Wi-Fi, or 3G) and aggregate them into a single backhaul for the software AP, too.