Cool! I was thinking about this very thing just the other day.

I think Andy and Ryan hit on the key things already. I went through a few questions:
What do we want evoke emotionally in potential users? A tagline shouldn't be a literal description, it needs to have feeling of "yeah, I want that!"
What makes Ubercart unique? Really, when we compare it to ALL the possible solutions out there, from a PayPal button to a Yahoo Store to a custom online store solution -- the integration with Drupal and flexibility are Ubercart's strengths
What do we want people to take action on?

Ubercart + Drupal is all about connecting the dots -- combining your physical product with your community, your customers, and things like multiple product formats or subscriptions. Lots of C-words here... commerce, community, content, combination, connection, customers...

It sparked any idea I'll throw into the ring here:

Connected commerce

The alliteration and balance between the words has a nice feel, and the "connected" is open to multiple interpretations.

Ubercart
Connected commerce