Ok, I think I have the problem figured out. There are 2 locations in the language files that the CHOOSE_ADD string is set. These are in the sendmessage.php and sendprivate.php language files. These must be identical, and because they are used in a form field they cannot contain any special characters (HTML encoding). If there is any difference between these 2 strings it will always try to use the address book entry even when none is selected.
Both of these strings should actually be in the generic file, so you don't have to worry about any differences. I'll get this changed for the next version.
