I was shocked to find that iMovie '09 would not allow storing event libraries on networked disks. The AFP volume would show up, but remain non-functional with a yellow exclamation sign.
There are plenty of instructions for hacking around the limitation with clumsy symbolic links. I discovered a much simpler approach, using an undocumented (?) setting built right into iMovie:"Allow Network Volumes".
Use at your own risk, try the following command in Terminal
defaults write -app iMovie allowNV -bool true