Smart phones and tablet pcs, like Ipads, are where it’s at these days. And it just going to get worse (or better, depending on your point of view).
Ecommerce sites should be readable and usable in both. If you’re shopping cart isn’t mobile compatible and your site isn’t designed to fit the screen it’s on, you’ve just lost some folks who won’t have the patience to try to read your tiny print or to scroll too much or otherwise mess with a site that’s too hard to navigate.
Here’s a Google article that talks about the subject. Yes, it’s got some techy language, but even retailers who don’t know a jpg from a server, will benefit from reading it.