Went and found the proper paper. It seems that the posterboard is being replaced by the recycled, toothy stuff I've been fawning over. I bought almost a whole box of it. It's a bit bluer and not as bright as the posterboard of old, but it makes for a great silkscreen surface, esp with the inks I'm using.
We went down to the studio last night and actually pulled 2 colors on 30 sheets in the intended colors...success.