With iDVD, you can render the result to a disc image (I don't remember the exact wording of the command). It's under the File menu as an option. It will do everything it usually does to create a DVD (all the rendering), but instead of burning it to a disc it will create a disc image. That image can then be used to burn DVDs using Disc Utility.
As already mentioned, if you've already created a DVD, you can create a disc image directly from it that should be identical to the image you would get from iDVD.