It is possible. In your home folder, there is a folder called Movies. If you've used iMovie, there will be a folder called iMovie Projects. If you've imported events, there will be an iMovie Events folder. Go into iMovie Projects to find the project files. Copy these to the Flash Drive or EHD. To back up Events, copy the iMovie Events folder. If not, you will not have video clips.
To work on the on another computer, put the files back in the same folders. If the folders don't exist, open iMovie. It will create them. Do not make them your self. The two folders actually have hidden file extensions. The full directory names are 'iMovie Projects.localized' and 'iMovie Events.localized'. Just be safe and let iMovie do it.
I have noticed that Finder in Snow Leopard occasionally has some issues copying files. You may get an error message something to the effect of 'the files cannot be read or written'. This is probably due to the fact that the project files themselves are actually folders with the extension .rcproject. No worries. Open Terminal (Applications -> Utilities -> Terminal) and type
'cp -r '
then drag the iMovie project file onto the window, drag the icon of your flash drive from the desktop to the window, then hit enter. All done.