I haven't noticed this so consistently with any other touch smartphone, but with both the models of Windows Phone 7 handsets I've had my hands on, I've had to be touching both the case and the screen for the screen to work as a touchscreen. Put it down on the chair, or the desk next to me and press the screen - nothing happens. Pick it up and hold it, touch the screen - beautifully responsive.
Am I unusually low on electrical current in my skin? Or unusually high? (I get shocked by static so badly and so often I press buttons with my knuckle rather than my fingertip to make it less painful.) I often can't press buttons on screen on the iPhone 3GS we use to navigate with in the car when it's in its cradle, although Simon has no problem - but it's not the all or nothing thing it is with Windows Phone 7 as I can sometimes do it on the second or third press. His hands are also much larger than mine, so perhaps he brushes the case at the same time?
Time for a test. The touchscreen on the Motorola Milestone I have next to me works when it's on the bed or the bedside table. So does the Nexus One. The iPhone 4 touchscreen also works when I'm not holding the phone, so I'm wondering if this is something specific to the Qualcomm platform all the Windows Phone 7 handsets are based on. I wouldn't call it a bug and I wonder if it might even be something that reduces the power needed for sensing touch, but I'm curious.
So, if you've got a touchscreen smartphone, especially if you have a Windows Phone 7 device, put it down next to you - ideally on a leather chair to exactly duplicate my test conditions ;-) - and tell me if the touchscreen is responsive. Is it me, or is it Windows Phone 7?
Incidentally, if you were thinking I meant attenuation; I made multiple phone calls with both Windows Phone 7 handsets and had no issues with the signal for voice calls, whether I was holding it at the top, at the bottom, resting it on the chair or balancing it against my coffee cup so the speakerphone picked up both of us at our separate desks. Humans are big, signal-attenuating bags of salty water and all the Windows Phone 7 handsets I've tried seem to have been designed to deal with this.