Don't bother with the import function - just copy and paste your clips across. Once you've imported using iMovie '08, open iMovie HD and start a new project. Then exit the iMovie HD application. Find the new iMovie HD project file (probably in your Movies folder -> iMovie Projects). Ctrl-click on it and select Show Package Contents. The Media folder that you now see is where you want to copy the .mov files generated by iMovie '08 during the import process to. When you reopen iMovie HD, it will tell you that there are a bunch of files in the trash. You'll need to move them out of the trash into the clip pane on the right.
Just be aware that you may have some issues depending on which transitions you use. After importing 1920x1080 video into iMovie HD once when I felt like experimenting, every cross dissolve was stretched - probably because there is no 1920x1080 setup in iMovie HD (the closest is HDV 1440x1080), and it was rendering using rectangular rather than square pixels. I'd be interested to hear if your experience differs (though I've long since given up on using iMovie).
Andrew.
