Pete,
That type of crack is not uncommon in some three piece bows. Could be cause by over tightening as mentioned already or even undertightening, or by moisture getting in at the limb bolt hole. I have had it happen in high priced custom bows that I purchased before starting to make my own and even in one of my own. If it is in the lengthwise direction as you stated, and it only goes an inch or two beyond the end of the riser,you are probably not going to have any more problems with it.
Now, I always laminate a .060" thick piece of micarta on the belly side of the limb and extend it to about one inch beyond the end of the riser. I feather the edge and it look pretty good and more importantly, no more cracks.
Long and short of it is that it wouldn't keep me from using it on a hunt.