Solarprobe, this is the Continental board (not Northwest). When you say you received Alaska upgrade certificates, I assume you mean they were received as part of your Northwest WorldPerks Platinum kit, not Continental OnePass.

Now to address the question posed by pjc, there is no reason to expect a complimentary platinum upgrade on the Alaska Air segments. If you really want an upgrade, you'll have to pay $$$. I would suggest visiting the following links:
http://www.flyertalk.com/forum/Forum8/HTML/000065.html
http://mileageplan.alaskaair.com/mp_paid.asp