This has happened to me many times with PC's & BB's, but I was surprised it happened to my new CDMA iPhone 4. I had a few apps running (Skype, Pocket Informant, Safari, & maybe a few others). It was in standby just sitting next to my chair. I picked it up and was surprised to find it dark and wouldn't turn on in response to either the home button or the standby button on top. I finally had to reset the phone by pressing the two buttons simultaneously as described in the manual. Fortunately, that rebooted it.
Anyone else had that happen?