My favorites of all the above suggestions are the two that Uberchic proposed in #34:
ubercart: where content and commerce meet.
ubercart: where content and commerce connect.
These bring together the notion that Ubercart encompasses more than just a cart, imply a CMS without using the acronym, and introduce the notion of Ubercart as a place rather than a thing. These share all the advantages and potentials of the "connected commerce" phrase Steph suggested in #3, without the burden of trademark infringement.
Or just say it straight out:
Ubercart: Because a store needs more than just products - It needs a community.