I came across this thread when I was searching for a way to remove "iPhoto Videos" from the event library some time ago. iMovie slows down to a crawl if you have tons of video clips in iPhoto and becomes frustrating to use. Here's what I found out.
When you "stupidly;-)" click "Now" instead of "Later" when it asks to make make thumbnails (wish there was a way of turning off this box), iMovie 08 goes into your iPhoto library and for every movie, it creates a thumbnail video in the corresponding "Data" folder (the original pics and videos will be stored sorted in the "Originals" folder.) In additional there is a iMovie.??? file (I deleted it some time ago so I don't know the exact extension) in the root of the Pictures folder which is probably a list of all the created thumbnail videos.
So to remove.
Navigate to where you have the icon of the "Pictures" folder in your "Users/????" folder. You have to right-click then "Show Package Contents" to actually see inside this folder. Now you'll have to do the same with the "iPhoto Library" icon i.e show package contents etc. Inside this folder you will see am iMovie.??? file, delete it. Now go to the "Data" folder. Go through every folder inside and delete just the thumbnail movie file. DO NOT delete the jpg file as that is used as the thumbnail for your events in iPhoto. Once you finished, load iMovie and you back to an empty "iPhoto Videos" events and the original spiffy performance. Now I just copy the videos I want to a folder on the desktop, import the movies into a new or existing event, do my youtube video, also Export a full size version for backup and I can delete the old events and projects in my Movies folder that I don't want any more. Neat and simple.