Thanks for your reply.
You are correct that setting the alpha for imported objects doesn't have any effect on export. I'm creating moving text within Motion so no media is being imported anyway.
I understand that motion is designed to work with FCP. I'm not sure I buy your assertion that Motion wasn't created to export video. I understand that I cannot import a Motion template into Premiere.
The choice to use Premiere is not entirely mine. I create these 4444 videos and send them to someone else who brings them into Premiere on their system. I much prefer Motion in terms of getting things done quickly. The decision to use Premiere for editing is the result of Apple's decision at the transition of FCP7 to FCP-X to make collaborative work difficult.
There is no green screen, keying or spill suppression going on. I tried doing some "matte magic" cleanup in Premiere and it helps somewhat, but I'm not going to send files to someone and say "by the way, you'll have to do some tweaky spill suppression in order for my files to look acceptable". Also, this is really just trying to minimize the symptoms rather than solve the problem.
The bottom line for me is that this used to work just fine, but Apple took away the option on export to choose Straight or Premultiplied Alpha. I assume there's something in the file header that indicates that information about the alpha channel but either Apple isn't tagging it correctly or Adobe isn't recognizing it correctly. My research indicates that Motion is exporting premultiplied alpha (in terms of video channel content, not necessarily file header) but when I examine the clip in Premiere it says it is straight alpha and I haven't found a way to over-ride that.
I would really hate to have to abandon Motion and relearn everything in After Effects just because of this one problem.
Here are a couple screenshots to help explain the problem. I created moving text in Motion with some motion blur. I've got it against a blue background to help make the issue visible, but the 4444 video was exported without the blue background. I then created the same blue background in Premiere and import the 4444 clip into the timeline.
Here's how it looks in Motion:
and here's how it looks in Premiere: