My understanding of the expiration date, is that it gets very hot in cars, and the plastic will begin to break down eventually. The expiration date is assuming that the carseat has been in the super-heated temperatures of the car, and is dated six years from the time of manufacture. Ever leave the kids' toys outside, and they get brittle at the end of the summer? Yeah. Car seats are made of sturdier plastic, but the car gets hotter, too. Thus, the expiration date.
Personally, I think it can be fudged a bit, but I certainly wouldn't ignore it. If, for instance, the seat spent about a year in the garage (NOT the attic - it gets just as hot there, as it does in the car!), not being used, then you can probably use it for 7 years, instead of 6.
However, check this link, for video of a ten-year-old Britax carseat. The straps broke free in the safety test. (spoiler alert: it's a TEST DUMMY)
http://www.youtube.com/watch?v=wvCRz7BRAM0