I'm logged on to the Internet (for $10) on Delta using "gogo internet", a WiFi service on the plane. So, I pulled out my WiFi tools to see what was going on.
Here is my speedtest. It claims I should be getting 1.7-mbps down with 128-ms latency, but subjectively it feels slower. As I'm browsing, it can suddenly stop and take many seconds for a website to appear. I bet that it's because the wireless connection to the ground isn't continuous, but keeps coming and going.
The network is 802.11abg (2.4-GHz and 5-GHz). Unfortunately, my tools only run on 'bg' adapters, but NetStumbler uses the 'a' adapter built into the laptop to show all the possible access points, as shown in the picture below:
There appear to be three access points at three locations in the plane (on three channels 1 6 and 11). I can tell they are at three spots because their signal strengths are different. I'm guessing they are in the front, middle, and back of the plane. These are Cisco access points that create multiple virtual access points for each physical access-point. Of these virtual access-points, one is open with a visible SSID of "gogointernet", the others are WEP and WPA encrypted and invisible. I have no idea why they are there. Notice also that we see the obligatory laptop with the peer-to-peer network "Free Internet WiFi" somewhere on the plane.
When I look at channel 1, I see a Blackberries and iPhones connected. I see these throughout the airport (along with Nintendo DSs and PSPs). I think these devices are automatically connecting to whichever access-point they can without their owner's knowledge. I walked down the plane and didn't see anybody with their phone out, so I'm guessing their phone is in their pocket/bag (and not turned off like they were asked).
If we look at the raw beacon packet, we can see that these devices are typical Cisco access points:
From a security point of view, there is nothing too interesting here. Like the inflight entertainment systems, the gogo WiFi service isn't interconnected with anything else in the plane, so there is no danger to the plane from this system being hacked. Ultimately, it's the same threat as any other WiFi hotspot (i.e. your cookies/passwords can be stolen if you don't encrypt everything).