I've made several phone calls since restoring my iPhone 4 with iOS 4.1 Beta 2, but I didn't manage to accidentally hang up on anyone. It feels like any of the proximity sensor troubles I've struggled with are finally gone.
Not every iPhone 4 seemed to have a quirky proximity sensor, but mine certainly did. I can't even count the number of times I've had to awkwardly explain that I hung up on someone because my ear or cheek touched my iPhone's screen at the wrong moment.
During a special iPhone 4 press conference a while back, Steve Jobs promised that these particular issues were not hardware-based and that a software update would fix them. From what I've observed so far, iOS 4.1 Beta 2 contains that mystery update. Either that or I've become particularly lucky all of the sudden.
Hopefully a public release of the iOS 4.1 firmware will solve the mystery - and the troubles.