As brulaz mentioned, I purchased a replacement white strip from the factory and installed it. It came with 3M VHB (Very High Bond) double sided tape on the back, which can be primed with 3M 54 primer for even better adhesion. I did not caulk it at all, and 3 years later, it is still on rock solid.
More info on 3M VHB tape -- it's designed to replace mechanical fasteners:
http://solutions.3m.com/wps/portal/3M/en_US/VHB/Tapes/
As you now know, the outer roof skin is NOT one piece. That plastic strip covers a seam. That seam had come apart on mine as well, and I glued it back together with (I think) polyurethane construction adhesive (PL brand).
Dave