I see the niche that mybitcoin covers as separate from a full-on shopping cart. Mybitcoin is a very good personal tool
for receiving and sending bitcoin. It wouldn't be so if it had all the functionality of a shopping cart.
The features of a personal send/receive tool could be written on the back of a napkin:
An essential element for mybitcoin, though, is a handheld interface. I don't know the status of this, but I don't think there is one.
P.S. This thread started over here:http://forum.bitcoin.org/index.php?topic=29055.0