I have the same issue described previously
http://www.pgadmin.org/archives/pgadmin-support/2007-09/msg00147.php
In the Query editor, items in the autocomplete drop-down lists cannot be selected by the Enter key,
it just enters (systematically) a newline in the editor window.
You must use the mouse (or the Numerc keypad).
This frustrating bug didn't appear in 1.6/1.7.
My version is 1.8.0, Win XP SP2
Hernán