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Any of the above variations can be used for mobile site. Though if you see that most of the big sites use m.sitename.com including Google so might want to use that one instead. As long as you can accurately detect the mobile device and 301 redirect to the mobile version of the site you do not have to worry about the duplicate content. Here is the link to Google blogs which talks about basic problems and solutions for the same while making a mobile website: http://googlewebmastercentral.blogspot.com/2011/02/making-websites-mobile-friendly.html

We program our cart in house and we need the SKU numbers to be passed. I looked at a lot of ecommerce sites and they all have their skus somehow int he URL in one way or another.