You can do it in Quicktime Pro with a bit of tinkering, but it's not free. Costs about $20.
...Or for a bit more money, you can purchase one of the Slick video effects packs for iMovie 6.
You can download iMovie 6 from Apple for free providing that you already have iMovie '08 on your system. The Slick video packs are available from www.geethree.com