All Windows 7 machines can become a wifi access-point, routing the connections over Ethernet or even over a client station connection on the same wifi adapter. This Slashdot article mentions this, but gets the facts slightly wrong (claiming that it's incomplete and that you need extra software). Instructions for doing this are below.
This is going to be bad, causing rogue access-points to proliferate in companies.
Technically, this isn't really new. You could always setup ad-hoc wifi and connection-sharing, which is almost he same thing. Also, it's already possible on Mac OS X, Linux, Windows Mobile, and iPhones.
Yet, a full "access-point" sucks less than "ad-hoc" networking. Also, it can work over the same WiFi adapter. Thus, while you are connected to "gogoinflight" on the airplane, your friend can log onto your "buddy" access-point on your computer and share your connection.
And there is increasing reason to do this. On my last flight, I wanted to sync both my iPhone and use my notebook. I only had to pay "gogoinflight" once, but I had to keep logging in again each time I switched from one device to the other. I totally would've just enabled this feature on my notebook and synced my iPhone through a virtual access-point instead.
Note: It only supports WPA, therefore you can't make "evil twin" access-points out of this (although I bet there is a way to hack it to turn WPA off).
HOW IT WORKS
Windows 7 can create "virtual" wifi adapters based on the real adapters, with a unique MAC address and everything. This is similar to VAPs on Linux, which allows you to create one virtual adapter for logging onto an access-point, and another for running a soft-ap. The difference with Windows 7 is that it creates only a single virtual adapter for "hosted" mode -- no matter how many actual adapters you have in the system. It's called "Microsoft Virtual WiFi Miniport Adapter", with the same MAC address decremented by one.
Making it work is simply a matter of (1) configuring the SSID and WPA password, (2) configuring Internet Connection Sharing to bridge it with the network, and (3) turning it on.
WHY IT WORKS
Zune, and stuff like it.
Microsoft wants you to be able to transfer music/video from your computer to your Zune easily. This makes it easier.
It's not just soft-ap. Windows 7 allows a lot of other low-level functionality. For example, you can write applications that add custom "information elements" to the beacon and association packets sent when new wifi connection is setup. Thus, your desktop becomes not simply an "access-point", but a "media access-point".
Finally, by mandating this low-level functionality in wifi hardware drivers now, it means Windows 7 should seamlessly work with "Wi-Fi Direct" bluetooth-like functionality whenever that standard becomes solidified.
STEP 0: Open a command-prompt with administrator privileges.
Click on Start menu, All Programs, Accessories, right-click on Command Prompt, select "Run as administrator"). Type in:
STEP 1: Configure the "hosted" interface:
netsh wlan set hostednetwork mode=allow ssid=Test key=letmein9
This example creates an access-point with an SSID of "Test", with a WPA password of "letmein9".
STEP 2: Configure Internet Connection Sharing (ICS)
Open up the networking control panel. Select the interface that currently has Internet connection (like your Ethernet or normal wifi), enable "Sharing", and then select the special "hosted" interface.
STEP 3: Start it
netsh wlan start hostednetwork
STEP 4: Enjoy
On your other devices (say, iPhone), connect to "Test" and give the WPA password of "letmein9".