Sorry I didn't respond sooner. I think I was reading the 'joined' date rather than the posted date and treated it as an old thread.
Anyhow, KAbay has outlined it very well. Spider cracks are typically due to surface stress, and unless they go down to the cloth layer, should not cause you any structural concern. I used a dremmel tool to widen the cracks prior to applying gelcoat for cosmetic reasons.
If a crack does go down to the cloth layer, and you see delamination, but no tears in the cloth, then a more significant repair is needed. If the area is not large, ie less than a 1/2 inch beyond the crack, I would clean it with acetone, and let it dry for quite a while. Infuse the separated layers with resin and hardner and compress the layers with clamps, weights, etc. Repair the gelcoat as indicated earlier.
If the cloth is torn, then a major repair will be needed. Sand or grind down past the damage and feather up to two inches on either side. Build up new layers of cloth resin and hardner, feathering/sanding progressively further out with each layer from the bottom to the top. Finish with a gelcoat layer, sand and buff to match the surrounding surface. Check this site for details:
http://www.westsystem.com/ewmag/19/Replacing_Core.html
I'm not sure about this next statement, but it is my belief that you see stess cracks on deck more often than you see stress cracks on a hull, due to two factors. The first is the thickness of the deck being more, thereby increasing surface tension as it cannot flex as easily. The second is that there are likely less stresses on any one place on the hull, as pressures are more evenly distributed.
I have a particular spot on one of my cockpit seats that is all spider cracked. It matches perfectly with where the end of the boom would come down in an unsupported drop, like when the previous owner dropped the sail. I have a number of other such inpact injuries along the gunwale where the rubrail would typically touch the dock. In contrast, I have no stress cracks on the foredeck, nor on the cabin sole, other than where the mast or blocks are attached.