In her Everything Wellness Blog, Dr. Toni Brayer has a good entry about how our culture still permits discrimination against fat people--even by physicians.
Be sure to read the comments too. One is from someone who lost 100 pounds and gained it back--and was able to observe the difference between how she was treated when she was fat and thin.
Obviously, wellness professionals have to be very sensitive to this. It's important to reach out to obese employees in a way that's inclusive and positive--but not discriminatory or blaming.
And of course make sure any incentives you provide in your program are just as achievable for the obese as they are for people with lower BMI. That's not just good practice--it's the law.