I have, several times. A few times I have let the guy in on it and used a ruller, a sheet of paper, the old trusted hand with the finger at the base (tummy side, not by the balls) and seeing how far up my hand it goes. Got caught doing that one time by a guy and offered to get out the tape measure and his eyes just lit up. He was just over 8" and was pretty pleased to know he really was above average.
24yroldvirgin has the right idea of how to measure a dick. If all men used this method I might start believing a few more about their size. (And my, such a nice dick he has too)
Last night I had an cyber old flame mgs me on yahoo and tells me he's lonely (aka: horney) so I talk to him a bit and he offers to show me his 9 inches. I bust up laughing. there is no way he could be more than 6 fully erect. Now how the hell did he manage to add 3 more inches? I was tempted to tell him you can't count the balls, but I kept my mouth shut.