I asked this question on Videofitness a while back, but I can't link to it right now for some reason, I'll try to do it later and post the link then.
Basically, I think that a pair of cross-trainers are better than either walking or running shoes for step, but mainly because they give you better side-to-side support. Walking or running shoes are both designed for you to be moving in a forward direction all the time, so they don't do much for keeping your feet happy when you are moving side to side.
I get shin splints every time I start walking outside on concrete after a long break. I think the main things you can do to help it are making sure that your shoes have enough cushioning and stretching and warming up thoroughly. You should definitely buy a new pair of shoes.... it'll just prevent tons of problems from all sorts of things. Also, one woman posted about shin splints on VF and suggested that you do "alphabet toes" to help them. Basically, all that you do is just sit down and point your toes and spell the alphabet in the air with your toes, trying to make the movements as big as possible. Doing them really helps me when my shins are sore and it will also help prevent the shin splints too.
I'll try to post those links later for you. Hope your legs are feeling better soon! I know how frustrating it is to be all ready to start up and then get side-tracked by an injury. Take care of yourself though... you really don't want it to end up being a chronic injury.
Amanda