Haha! I'll have to remember that trick where the DM becomes a player and an unsuspecting player becomes the DM
I've done that in a campaign, but it was pre-arranged. My friend, Jeremy, was DMing a high level game where the players had decided they had to planar shift from Mysterra (sp?) to Greyhawk to find the McGuffin. I had been DMing a different game in Greyhawk at the time. When they teleported Jeremy and I swapped places. He took over my character and I sat behind the DMs screen.
Jeremy ran rather a heroic game where characters always eventually won out. I ran a brutal game where death stalked behind every corner. The players were horrified that suddenly their favourite characters were at real risk of death. They didn't die but it was a close thing. The immediate increase in tension was fantastic and the players loved (and hated) every moment.
I'm still a brutal DM - and all my dice are rolled out in the open. The death count in my current campaign that has been running for a year is 4 deaths and one almost TPK (it hinged on one dice roll).