I obtained a 3-month business visa before traveling to Phnom Penh mid July. The visa expiration was listed as Sept 10th. I just extended my visa for a year and was told by immigration that I had overstayed my visa by 16 days @$5 per day. The agent says everyone entering the country gets only 30 days of visa coverage, even though the visa obtained in the US said it expired Sept 10th. They just don't tell you that when they stamp your passport at the airport!