I'm pretty sure this is true. If I fix a flat with co2 I always have to pump the tire up the next day. I wouldn't say they get totally flat, but pretty squishy. Still I love having cartridges over a hand pump on the road. I still carry a really small pump for emergencies, but there's nothing like changing a flat in 40 degree rain and having to pump and pump and pump with a little hand pump just to get 70 lbs of pressure. Blech - co2 is so much quicker.

Even plain old air leaks out slowly, if I let my tires go a week without pumping them up they'll go from 100 lbs to 80.. its just not as much or as noticeable as the loss with co2.