It's been running around the net for some time now but I never really got to chance to check it out. I don't know if it's only for B&W or if it applies to color too... or if it's just a hoax...
I know the expiry date is on there for a reason... but it's not like yogurt

I think it might have something to do with the earth's ambient radiation which increases the film's fog level which could render it less sensitive... or at least that's the reasoning I put behind it. I must say this isn't an exact science and it depends a lot on how the film was kept.
I think Sean (moominsean) could be better at explaining this phenomena. I know he has a lot of experience using very expired emulsions with quite good results.