It happened to me on my first phone call with the new Apple iPhone 4: The display screen flashed on during the call, and I managed to inadvertently put the call onto speaker. Twice.
Now, I could crack a joke about having a talented cheek, but this isn't a joking matter: I never had these problems with my iPhone 3G or iPhone 3GS. I didn't feel as if I was holding the phone any differently; I even paid close attention over the course of subsequent e-mails, and confirmed I wasn't doing anything different.
What I did notice as the weekend wore on, however, was this was not a one-off occurrence. I regularly activated the touchscreen during a call. Typically, I managed to activate the keypad (and subsequently dialed numbers), mute button, or speaker; sometimes I ended up going into the contacts screen, or activating FaceTime (which in turn gave me an error message, given that I wasn't on Wi-Fi).
Prime Suspect: Proximity Sensor
The clear suspect in this bizarre behavior appears to be the iPhone 4's proximity sensor, mouthful of a term that describes the sensor that detects your face's location relative to the screen, and enables or disables the display accordingly. On the iPhone 3GS, the proximity sensor was located to the left of the earpiece speaker. But that space on iPhone 4 is now occupied by the front-facing camera, and the proximity sensor is above the earpiece.
What's not clear is whether the iPhone 4 screen's misbehavior is due to the new location of the sensor, or it's because Apple tweaked the sensor's responses in any way. It could even be a combination of both.
In use, I observed out of the corner of my eye that the screen would blink on and off intermittently, depending upon how I held the phone. It was almost an assumption that I'd see this behavior if the phone slipped just slightly away from or up from my face. And it happened consistently whenever I rested the phone between my head and my shoulder-a common position, albeit one my neck never particularly appreciates.
(Separately, the new proximity sensor location is a major reason why you shouldn't attempt to use an iPhone 3GS case while waiting for an iPhone 4 case, even though the latest-gen cases are almost as hard to find right now as an actual device. It's also one of the reasons why many case designs remain in development and are not yet available, according to Ramsey Oten, CEO and case designer for Sena Cases. Notice that the earliest designs are either pouches, or form-hugging designs like Apple's own bumper, and similar designs from Sena and Incipio.)
While little official is known about this issue yet, I found it incredibly annoying to have my calls routinely interrupted. I asked around and found my colleague, Ginny Mies, had similar experiences. And some digging online shows Apple has an open discussion thread running 19 pages long, and counting. There, a user reports that an Apple Store Genius tech said it was "probably a software issue" but still put that user in line to swap out the phone when they get more in.
Sadly, I wouldn't get your hopes up on a swap helping matters. I'm already on my second handset, and have experienced the proximity sensor problem with both. Each was a "clean" install, meaning I didn't restore from a backup or anything else that might have impacted iOS 4's settings. To be fair, that first handset had other issues, too--the phone app froze up twice, each time requiring a reboot, and multiple times, the touchscreen didn't respond, period (among other things). But shortly into my second call with the second handset, the proximity sensor problem kicked in again, and I activated the keypad. In my experience, I'd say it's not an isolated hardware issue.
In these early days, it's not clear if every handset is affected--heck, many users have gone straight to using a Bluetooth headset or haven't really used the phone for conversation. Nonetheless, it is clear that this is yet another iPhone 4 launch problem Apple needs to address. Soon.
In fact, I'd put this call interruptus problem right up there with the reception issues. Yes, like so many others, I can hold the iPhone 4 in the so-called death grip and watch its signal strength deteriorate bar-by-bar, but I have not dropped calls because of this problem; I just drop calls in the same locations where my iPhone 3GS always dropped calls. At least the reception issue can be solved by using a case, something most of us will do, anyway, once cases become widely available. But short of using a headset--which no phone should require--the continuous and awkward call interruptions appear unavoidable until a fix comes along.