The Department of Labor explains the rules here. Wellness programs that don't link rewards to "health factors" are fine; for example, you may reward participation in a smoking-cessation program, so long as participants aren't specifically required to stop smoking. But if rewards are tied to "satisfying a standard related to a health factor," certain conditions must be met. The Department of Labor's website provides more detail.
If you question your program's compliance with HIPAA, you'll want to read the applicable law carefully and seek appropriate counsel. But from our perspective, the law is on our side. Rewarding participation is not only firmly within HIPAA's guidelines--it's the best way to motivate your employees to join the program and get healthier.