Based on your price range i say the safest bet is to get the S400, but others to consider would be the olympus stylus 400 which is all weather proof, and also the pentax optio S, which is even smaller than the s400. if your out going, give the olympus a try, its not that much bigger than the S400, but since its all weather proof, you would probably be more incline to use it under conditions that you might not with the other two. If you love size, and size only, i think the pentax is pretty small, and altho its mp count is lower than the others, 3.2 should be more than enough if youre not going to print more than a 8x10. Lastly, again, the safe one is to go with the canon, they were the first to introduce small cameras like these, and have had plenty of success with previous model. Any of these three i would recommmend, but i say, go to henrys, or futureshop, buy it, try it, and return it if you dont like it.
hope this helps