From what I've read, the theory is that the hormones in birth control pills can increase the cholesterol levels in the bile, which can cause gallstones. Maybe the types of hormones in Yaz cause this more often than other birth control pills.
After all I've heard about it, I decided to go off Yaz, just as a precaution. I never had any side-effects, but I'm worried that I will. I'm hoping the FDA will look more closely at this and come up with some real findings, rather than our having to trust class-action lawyers. Until there are some real scientific findings, doctors aren't going to take this seriously and stop prescribing it (mine didn't think this was any big deal, I had to insist that I wanted to switch).