Yeah, most of the time they just come when they're good and ready.
Mine were both 42+ weeks. I have read that the 'real' gestational period for humans is actually more like 41 weeks on average, so that about.com article might be accurate?
I did also read that girls tend to have shorter gestations than boys. One study I read the abstract for reckoned that correcting for girl twins, the average gestation was around 41+3 weeks.