This is a common "problem" with IK animation. One way to get around it is to set your FK/IK Mixing to something less than 100 and create an FK key at frame 0 where the bones are bent. Another way to fix it is to make sure the leg is never stretched 100%
IK Constraints aren't done yet and there are some bugs so Mixing might not give you the result you want. This will be fixed when Nate is back from vacation in about a week from now.
