Great news! After some browsing around, I located the problem, and now my movie project is fixed!
As it turns out, videos that are located in iPhoto are referenced based on the Event that they belong to, which means that if you change the name of the Event that the picture belongs to then iMovie will no longer be able to find your picture. (This is also why the clips appear in the lower section of iMovie, because they are still there, just in a renamed or merged event.)
I was able to fix the problem very easily by renaming my event back to the original name, and then after closing and restarting iMovie all the nasty little yellow triangles were gone and all my clips were back as if they'd never left.
This may or may not help, but for me, to make sure that I got the event name correct, I found the movie file (which was in "Movies/iMovie Projects") and then right mouse clicked and selected "Show Package Contents" so that I could see what was in my project, and then opened the "Project" file, which had the names listed in it. Hope that helps!
Trigby