Michael Strain from the American Enterprise Institute helpfully offers this chart indicating that despite a wealth of anecdotal evidence, there's no hard statistical backing for the theory that the Affordable Care Act is leading employers to cut back workers' hours:
My strong suspicion is that if the ACA has an impact on the labor force (which it probably will) it will be through a different mechanism. Right now "in order to qualify for health benefits" is a very good reason to work full-time, even if you'd rather have more free time and less money at your current wage level. The Affordable Care Act will make this benefit qualification rationale less compelling, and for families who are within the range for ACA subsidies it will also make the trade of more free time for less money more attractive. Which is to say that rather than making employers more reluctant to hire, the ACA is likely to make workers more reluctant to work—especially more reluctant to work long hours for meager wages in unpleasant conditions.
Obviously you can look at that as a feature or a bug according to your taste, but it should have a discernable impact on macroeconomic aggregates when the law is fully in place.