desktop-clear-preserve-buffers is a variable defined in `desktop.el'.
Its value is

("\\*scratch\\*" "\\*Messages\\*" "\\*server\\*" "\\*tramp/.+\\*" "\\*Warnings\\*")

List of buffers that `desktop-clear' should not delete.
Each element is a regular expression. Buffers with a name matched by any of
these won't be deleted.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.3 of Emacs.