http://code.google.com/p/yasnippet/issues/detail?id=23

--- a/dropdown-list.el
+++ b/dropdown-list.el
@@ -69,7 +69,7 @@
   "*Bla." :group 'dropdown-list)
 
 (defface dropdown-list-selection-face
-    '((t :inherit dropdown-list :background "purple"))
+    '((t :inherit dropdown-list-face :background "purple"))
   "*Bla." :group 'dropdown-list)
 
 (defvar dropdown-list-overlays nil)