What's your OS?
On windows and Leopard it's easy to use Print Booklet from the File menu to print to PDF as an imposed file. For later versions of OS X ist's still possible to print to .ps file and distill.
Dave Saunders wrote two scripts I use for imposing, but sutability will depend onthe document, whether you have objects crossing the spine, and if you need creep.
Clint supplies files in reader spreads, wants final PDFs in printer spreads also
native files (collect/package) the InDesign back in printer spreads. In case
there is a change at the printer. Right now we are just moving the pages around
mnual, keeping in mind any cross overs. This all works for us just thinking there
might be a script for this.
Mac OS x 10.6.8
See my reply above for my reasons.
I hope you're getting a real bargain price on the printing for doing the impostion. I'm guessing creep is not an issue, so for files without crossovers you can use this one: http://pdsassoc.com/downloads/Buildbooklet.zip and for files with crossovers you can try Dave's CS3 Booklet Script
The first makes a shffled copy of your doc and you need to export spreads. The second imposes the pages by placing them into spread width pages.
That works great! One click and its done.