I have written a simple module which adds a Simplenews newsletter subscribe checkbox to the Ubercart checkout process. This module is tested and works with Ubercart 6.x-2.0-rc6 and Simplenews 6.x-1.0-rc6. It uses an combination of hook_checkout_pane() and simplenews_subscribe_user() to achieve the objective.
It's very basic in the sense that it will subscribe the user to one newsletter (the primary newsletter), and if the client is already subscribed the checkbox still appears. It works for both anonymous and user logged-in checkouts. However the user is subscribed when pressing the 'Review Order' so if they review their order, then click the 'Back' button and untick the box, they will still be subscribed Not ideal, but it's fine for my purposes.
I think the module was a bit too simple to put on the main Drupal site (my application for a CVS account was rejected so I'll put it here instead.
Hopefully others will find this useful.