No it is not possible, unless you out to an HD capable monitor. It should be able to edit HD video, however.
To respond to you cry of "BS", portables could play HD, in theory, but it would take such an expensive display that I doubt you or anyone would want to buy it.
iMovie HD
" Mac OS X 10.3.4 or later
QuickTime 6.5.2 or higher
400 MHz PowerPC G3
256 megabytes (MB) of RAM (512 MB of RAM required for HDV support)
2 gigabytes (GB) free hard disk space
A display with at least 1024 x 768 pixel resolution"
1. download the HD trailer to your local hard drive.
2. using QT Pro, export (transcode) the trailer to the DVCProHD codec.
3. the DVCProHD version of the trailer should play back perfectly smoothly for you. DVCProHD will take up a fair amount of disk space, so maybe have an external hard drive handy if you're collecting.
Try it and tell me how it goes. Your PowerBook should play back DVCProHD smoothly. (command-I to see the framerate during playback in QT 7).