I'm sure, like most products, the treadmill is over-designed. If the weight limit says 300, you could probably put 600 or 900 lbs on it before it actually broke. 320 lbs will put more wear and tear on it than 120 lbs will, but really not substantially more wear and tear than 300 lbs would. I say go for it.
I'm glad you read the instructions though. I had friends who bought a (Walmart) trampoline, put it up, started using it. It wasn't until I started researching them myself that I realized that most of them have a fairly low weight limit, and it wasn't until you got to the really high end trampolines that an overweight adult could reasonably use one. They had 5 or 6 overweight adults on it at ONCE and sure enough it broke. (Actually, it's not why I told the story, but it does highlight my point about over-designing.)
First mini-goal: 5% - 209 - Met 04/29/11
Second mini-goal: 10% - 198 - Met 6/27/11
Third mini-goal: 15% - 187