I've spent barely any time with iMovie '08 so far (I'm slammed with projects, and Apple's timing for iLife '08 didn't help any), but it's pretty clear that this iMovie is not the same beast that we're accustomed to. A few very preliminary notes:

• This advice has always been true, but it's even more important now: If you're working on an iMovie project in a previous version, finish the project in that version. iMovie '08 can import iMovie HD projects, but it seems to be just a way to grab the raw video files. You'll lose transitions, etc.
• iMovie '08 does not appear to have any support for third-party plug-ins. This version really is a completely different program that got named "iMovie."
• When you install iLife '08, the previous version of iMovie HD stays on your system instead of being overwritten, as in versions of the past. If you're more interested in production value than quick-and-easy editing, you'll probably prefer to work in iMovie HD.
• Some features you may be accustomed to are no longer present, such as DVD chapter markers and bookmarks. Gone too are themes, one of the major new features in iMovie HD 6.

More as I play around with it.