The timeline is missing, but if you are doing short burst videos (which 08 excels at) it's not really needed. If you are doing longer movies, you might decide you need it, though 08 has very precise control tools and lots of feedback on the timeline in 08, and I find I'm not missing the timeline. It does, however, require learning some new ways of doing things.
As for separating audio, you need to do that in another tool.
Here's an example: I had a 3 minute video of my daughter singing at a special Christmas event, and I wanted to take that video, keep the singing audio throughout , but cut in video clips (without their audio) of her doing other things leading up to, and after the choral event.
I saved the choral audio track using QTPro, brought it into 08, along with the original video (thereby seperating the video from the audio), and then dropped in short clips of the other videos I wanted. It was very easy to align the clips where I wanted them to be by simply looking at the time and frame counts iMovie 08 displays as you adjust the length of the video. Total production time was less than 10 minutes. '08 exceled at this, because I can dynamically see where I am in the clip as I size it / shrink it in the main video stream. I actually started off trying this in 06, but became frustrated because the interface was not dynamic enough. Cropping the short burst videos I wanted was too troublesome (crop-review, crop-review, crop-review. Ugh!), and I gave up. For some things, particularly around shorter videos with basic effects, '08 is frankly superior in ease of use.
Overall, I think the interface for 08 is a HUGE leap forward in design, but like elppa said above, I think there will be some kind of blending of these concepts into the next gen release.