Most sites will snatch up any TLD that they can to protect their brand, whether they use it for the specified purpose or not. I think sticking to the '.com' is important for a brand, but we'll see.
The 'm' subdomain is pretty standard right now:
Amazon.com uses a sub-folder for mobile devices (
http://www.amazon.com/access), and so does Kayak.com (
http://www.kayak.com/moby).
Lots of airlines use the
mobile subdomain.
Many sites also use an auto-detection of the device to serve mobile content, like
http://shopping.yahoo.com, when a mobile stylesheet isn't enough. Quite a few are still using
wap or
wml, like
http://wap.ebay.com or google.com/products/wml (Froogle).
Google has applications that you can download by pointing your device to mobile.google.com, which includes search, maps, gmail and news. This might be an alternative for larger online retailers.
Another cool feature that some larger sites are beginning to promote is searching via SMS, like
http://www.google.com/mobile/sms/index.html, which cuts out the need for a 'domain' altogether. A shopping site could have a number that you text your query to, then you can purchase the item using PayPal mobile, or equivalent.
One of the most exciting and relatively new 'mobile shopping' ideas is for phones with built-in cameras, where you'd snap a shot of a barcode on a product, or a special barcode in a magazine, for example, and comparison shop or purchase right there.
http://scanbuy.com is doing some cool things in this arena, specifically with it's
http://www.scanbuyshopper.com service.
It's exciting to see mobile evolving, for sure.