As far as I know, only iMovie '06 is available for download. The reason for this is that iMovie '08 is a complete re-write of the program and, although new, drops lots of features that were originally present in '06 (such as special effects). For many users, this was a 'downgrade', but when upgrading from iLife '06 to iLife '08, iMovie '06 gets deleted (Intel Mac users only).
After numerous complaints, Apple posted iMovie '06 online so that those who didn't want or like the new '08 version could still use the old one if they preferred.
Personally, I've not tried out '08 yet, but will do when my new MBP gets here this/next week. My new-to-Mac dad is raving about it on his new iMac...