The Dual CD can be used to install a full KDE or Gnome desktop so long as additional media are declared. If additional media are declared then the package selection screen looks like this http://dl.dropbox.com/u/285824/dual_with_media.jpg  The user can choose a KDE,Gnome, or custom install. If custom is selected then a screen showing detailed package selection is shown.

If no additional media is declared the screen to select KDE or Gnome is skipped, and instead the user is presented with the custom install selection screen
http://dl.dropbox.com/u/285824/dual_no_media.jpg
Here LXDE is already ticked but KDE, Gnome, and all the other selections may be ticked by the user.

If the user ticks KDE or Gnome but leaves LXDE ticked there is no harm done. Installation proceeds as normal. LXDE is installed, KDE or Gnome are not installed and the user may wonder why, but at the end of the installation they have a working system.

If however the user ticks KDE or Gnome and unticks LXDE, then installation continues but there is no DM installed and the user boots to the command line.

I suggest the behaviour should be to alert the user that additional media are required if (s)he attempts to tick anything other than LXDE and put them back to the define media page.