This is true - however, it depends which border crossing you use. You will get a visa at any of them, it's just a matter of how much they'll make you pay for it. When using the land border crossing points from Thailand, Cambodian immigration will issue you the visa (official fee 20 USD) for 1000 Baht (25 USD). Guess who gets the extra $5? If you happen to share my aversion to stuffing the pockets of corrupt immigration officials, get your visa in advance. Or fly into Cambodia, at the airport you're charged the 20 USD you're supposed to be charged.

BTW, flying into Cambodia has become a lot cheaper since Air Asia (www.airasia.com) and Jetstar Asia (www.jetstarasia.com) are now operating routes there (from Kuala Lumpur and Bangkok on AirAsia, Jetstar from Singapore). Tiger Airways (www.tigerairways.com) can also be useful when travelling around Southeast Asia.