Unlocking iMovie '09 & '11
This is a really nice tip for those needing slowmo effects in iMovie '08, but don't want to send their project to iMovie HD. (If you haven't noticed, I don't really ever post tips that say, "Do it in iMovie HD." Where is the fun in that?)
Anyway, Karsten Schluter, one of the most helpful and insightful regulars on Apple's iMovie '08 Discussion boards has posted a great walkthrough for creating a slowmo effect with a free application called JES Deinterlacer.
SlowMo with iMovie '08
All I would add to Karsten's instructions is that you can just save the slowmo clip in the correct Event folder and iMovie will automatically find it and create thumbnails for it the next time you launch iMovie.
And by the way, Karsten, if you read this: That demo clip is A W E S O M E. What a kick!