A recent visit by Unesco officials to Preah Vihear prompted a curt notice by Thailand that Unesco must ask for permission from Thailand before any visits to disputed areas. However, the Unesco delegation said that they steered clear of any disputed areas and only toured the Cambodian side. It sounds like Thailand is touchy over the (unresolved) territory issue at Preah Vihear, and it also goes to show how much injured pride is at the heart of this border dispute.
Thailand to UNESCO: Ask before entering disputed border zone
MCOT News, 21 May 2009