Open System Preferences > "CD's and DVD's" and tell OS X to open the Finder (instead of iDVD) if an empty DVD gets inserted.
If you insert a DVD, there should now be an icon on your desktop that acts like a normal folder on your harddisk. If you try to eject the disk (with the Shortcut Command-E or by dragging the Icon to the trash bin in the dock) a dialog box should appear, asking if you want to burn the DVD.
At least that's how it works in Panther, details may have changed. Oh and remember that your system partition needs up to around 5 GB of free space for trouble-free usage of this feature.