I can't remember how it used to be done, but what happens if you just hit 'burn' again? Is all your media already encoded? If so, it should just burn another copy.
Alternatively, you can use the 'Disk Utility' to make copies. You just make a master copy that lives on the computer, then make exact duplicates of this with the Disk Utility. I've used it quite a bit for small runs and it works perfectly, including all menus and so on.
Here's a tutorial from Ken Stone : [
www.kenstone.net]
This is also a great backup for your DVD projects - you can keep multiple projects on a single disk or external, so when the cleint comes back three months later, you're already set to go with more copies.