The Verge. Kinect Play Fit will track exercise across "most Kinect games," providing metrics that'll be stored in the cloud. Joule is expected to be a heart-rate monitor, allowing wireless monitoring to keep exercise "within a target goal for weight loss, strength, or cross-fitness workouts."
Joule and Kinect Play Fit are compatible, but are two separate products. There is no expected launch window, but Verge sources also claim a future dashboard update will introduce the system. We're checking with Microsoft if it has any comment, but most of this (if true) will likely be revealed at E3 in a month.
Update: "Microsoft does not comment on rumors or speculation."