Update: October 2001.
The soon-to-be-released version if iDVD 2 will have the watermark optional.
At least they listen!
For reason known only to them, Apple decided that iDVD should place their watermark on the bottom right of all menu screens. What liberties! I don't see any other package placing logos automatically on their output! Imagine if Adobe placed their logo on every PhotoShop document, or MicroSoft placed their logo in the header of every document. Fortunately it's relatively easy to remove although not as easy as an option in the preferences would have been. This document will describe how. To remove the watermark yourself (or to add your own watermark) you will need PhotoShop. If you don't have PhotoShop don't worry, I've provided a solution for you at the bottom of this page. The following has been tested on version 1.0 of iDVD.
The watermark resides in a file called "Parts4" within a folder called "Interface Files" within a folder called "Resources" within the "iDVD" folder. This file is in PhotoShop format even though it has had the type and creater settings zapped. At this stage please make a copy of this file in case something goes wrong.
Open the "Parts4" file in PhotoShop, you will need to choose the "All files" option and select 'PhotoShop" from the formats menu since the file has the wrong creator/type. The file consists of a large number of layers, turn on the layer called "name:watermark type:Tile" and make it the active layer, turn off all other layers. Delete the existing watermark. iDVD expects something on that layer. I created an almost white pen (254,254,254) and made it almost totally transparent (1%) and drew one pixel. I can't see the resulting watermark. You might like to add your own watermark here but I suggest it is more appropriate to place such symbols on a custom background graphic. Save the document, it doesn't matter that it now has the PhotoShop creator/type set.