I'm also interested in integrating Ubercart with Drupal's "Signup" module.

One key issue is how any solution handles the purchase of multiple tickets/registrations. For instance, if a user wanted to purchase four tickets to an event, you wouldn't want to require them to make four individual credit card purchases (which might look like invalid repeat charges to a credit card company). Instead, they should be able to put four tickets in their cart, buy them, and then be directed to the Signup module to register each of the four individuals in sequence.

To help manage all of this, I'm wondering if Drupal's "Workflow-ng" module should be involved in any potential solution.