Ars Technica has a really interesting article about the fabled proximity sensory in the iPad 3G. I know what you're thinking: "Why does the iPad 3G have a proximity sensor?" Well, if Ars did a proper job at finding and interpreting FCC papers (as I believe they did) it's there to protect users from electromagnetic radiation that emanates from the iPad's 3G antenna. Since distance affects how much radiation is absorbed by a person's body, the iPad 3G's proximity sensor is there to dial down the power of the 3G radio by as much as 75% when it detects that a user is close to the sensor. Less power means less radiation exposure. It also means a weaker 3G signal and slower data rate.
However, the 3G signal isn't only dialed down when the proximity sensor comes into contact with a human user. In fact, anything (like an iPad case) will cuts the 3G radio's power. That's why some iPad 3G owners report weaker 3G signals when using their iPad with a case attached.
But now a company called Pong Research has come out with a US$100 iPad case that it says will enable your iPad's 3G data to run at full speed even when the case is on. The case accomplishes this via a small hole right where the proximity sensor is located on the iPad. As Ars points out, any case manufacturer that knows about the location of this unadvertised proximity sensor can do the same. However, by just cutting a hole near the sensor you still have a radiation problem. That's where the second big feature of Pong's case comes in: an integrated circuit board designed to take that harmful radiation and redistribute it away from the user's body.
Arstechnica tested out both claims (the redistribution of radiation and the boost in 3G signals) and found that the case works as advertised. So if full 3G signal and a lack of pesky radiation is for you, go on over to Pong's site and order a case. If you hadn't heard anything after I said you iPad emits deadly radiation, check out Ars's article for an in-depth description of consumer electronic radiation (hint: you'll be fine).