I've been having the same freeze-ups when I downloaded the new iMovie software from iLife '08. easy fix...I deleted the entire new iMovie siftware, re-downloaded the iMovie siftware from iLife '06 (that I've been using successfully), donwloaded the patches (3 of them on the Apple site)...and now all is well and back to the smooth-sailing as before.
I Do Not Like, nor want, anything to do with iLife '08.
(I love Apple).
best from Baltimore City, Jay