LBSes are NOTORIOUS for overtightening pedals...actually, so is everyone else...people get these pedal wrenches that are like, 6 feet long, and they crank down on them SO HARD so that no ONE, EVER will be able to get them off...

anyway...i usually just keep trying to get it until i get discouraged, i let it sit for a while, then come back to it when i'm feeling better...

make sure that you're turning the wrench in the right direction...maybe try some sort of thin lube...if you can't get them loose, take it to a shop, and have them just remove the pedals. put the new ones on yourself with a short pedal wrench, and tighten it just past snug. and make sure you make them thar threads nice an' greazzy!