In CS3 you can create booklets on printing, so I guess Adobe didn't see a need to save one. Print shops prefer to do their own imposition anyway.
If you need to have one for some reason, you can print the booklet file to the Acrobat driver and make an imposed PDF file. Of course, you'll need to go back to the Indesign file to edit it. Or you can make a second InDesign file that has pages twice as large, and import the Indesign file into it page by page, laying it out appropriately. Make your edits to the original file, and the imposed file will update automatically.
I've always liked to save the page layout booklet for printing later. We have our own print shop here at our school. CS2 was so much better with this. Adobe has probably had a lot of complaints about this.