My present work around for the lack of a Save command is to do all my work off of a FW drive. Using the Project window I drag a copy of my iMovie project to the FW drive as a back up and work on that one for a while. Then I drag it to my internal drive and work on it there. Always, there is a recent back up on the "other" drive. All of the support files stay on the FW drive so I can use it on a third computer--and have yet another recent back up.
All this, however, would not be necessary had Apple not idiotically and callously removed the 30 year-old File>Save command from this application. That command is perhaps the only common command in all software--Mac, Windows, Linex, BeOS, you name it. Apple, in its nauseating drive to force people to Final Cut Express, sliced it off. It probably took them forever to find it in the code.
Although it may not help much, write Apple how absurd it is not to have a File>Save option--as every piece of software in the last 30 years has had.
In another post, I mentioned that the lack of a Save option in iMovie was inexcusable and was hauled over the coals for such a suggestion. So be prepared for some ignorant people to disagree that we don't need to save our work nor need to pick where to save it.