"If UA have not bought you a fare with a bundle (baggage) you are out of luck."

I checked again with United. They say they do have a baggage agreement in place and the baggage will be tagged all the way to ADL. So technically, we won't have to check our luggage again. On the Jetstar website, at least as far as baggage are concerned, there seems to be an agreement with United, see the list of partner airlines at the bottom of the page: http://www.jetstar.com/us/en/plannin...gage-transfers

This being said, my experience flying two separate airlines on the same ticket has been that I needed to get at least my boarding pass at one of the second airline counters or the gate. Often (but not always), they replace the tags on the luggage at the same time to use their own system... and that's where I am afraid things could get hairy.