I sell Mitsubishi Projection TVs. All the current (year 2000 to present) Mitz Hi-Definition series Projection TVs specify in their owners manuals that you do NOT, REPEAT NOT, leave the broadcast standard image (4x3 aspect ratio) picture on a Hi-Definition type TV (16x9 aspect ratio)for more than 25% of the time. You WILL incur phosphor burn-in along the line of the border if you do so.
I agree that the absence of light (or in this case energy), will not cause premature damage to the luminescence coating on the interior of the picture tubes. The problem is: That by consistently blacking out the sides (or top and bottom)of the image you will continuously saturate only a portion of the picture tubes phosphor coating. This partial area saturation will prevent a "consistent degree of burn-in" to the entirety of the screen; resulting in edge lines that will eventually become obvious to the eye. Once these "borders" are burned into the phosphors you will NEVER be able to erradicate them and you will be forced to live with them for as long as you own the TV. In my professional opinion, watch the stretched image most of your viewing time. I personally wouldn't want to gamble away 10 years off the lifespan of a $3000 TV for 15 months of inconvenience.
You will only have to live with this for a couple of years untill Hi-Def becomes the standard TV broadcast format. The manufacturers have told us they expect about 50% of all broadcasting will be done in Hi-Def by next spring. And the Big Guns in the industry, like Sony, Mitsu, Hitachi and Toshiba, will start mass marketing stand-alone Hi-Def Processors for Cable, Satelight and RF Broadcast starting this summer. And the competion for the market should drop the current $600-$800 processor price dramaticly over the next year and a half. This processor will greatly enhance your viewing options and you'll be able to see the knock-out picture that the HD sets were designed for. I watched the Winter Olympics in Hi-Definition last month and it was breathtaking.
So hang in there for a few months. You've got a great TV - don't screw it up.