The following TripReport snippet is taken from 2003/12 Merlin - Spending the Holidays In Cambodia

Currency

There were no ATMS in Siem Reap, but there were money changing booths that take traveler’s checks, cash, Western Union, etc.

The de facto currency is the U.S. dollar, take plenty of ones and fives. They also took Thai Baht at a lot of places as well, but be careful because the vendors like to screw with the exchange rate. Any change less than a dollar is given to you in the Cambodian Riel. Riel is cumbersome because you need a whole wad of it to may for things since $1 equaled about 3800 Riel.

Many of the restaurants and hotels took credit cards, mostly VISA and Mastercard; fewer took AMEX.

The following TripReport snippet is taken from 2003/12 Merlin - Spending the Holidays In Cambodia

GETTING CASH: Although there are no ATMS in PP, money is readly accessible. There are several banks in PP that give cash advances. The best bank was the Cambodia Commercial Bank, near my hotel, which charges 2% on Traveler’s checks and Credit Card Cash Advances. Don’t forget to take your passport.

In Sihanoukville there was a Chinese bank on the main street in downtown (I forgot the name of the bank) that had no surcharge Visa cash advances. It had large Visa logos in the windows with "no surcharge" prominantly posted in English. I got a $300 cash advance there and indeed there was no surcharge or cost to me. (other than anything your bank may normally charge) MasterCard was NOT accepted at all there so bring a Visa card. And yes, I did forget to take my passport :-) and had to make a trip back to the hotel to retrieve it from the safe deposit box. Other than that the whole process took less than 5 minutes.

In Koh Kong, I was unable to detect any way to get money. As far as I could tell, whatever you arrive with is what you have and if anything happened where you run out or it gets stolen, I guess you would be walking back to the Thai border where you could get money in Hat Lek Thailand. Koh Kong is also unusual for Cambodia in that prices are always quoted in Thai Baht although US dollars or Cambodian Riel are readily accepted everywhere as well.

I found the name of the Chinese bank in Sihanoukville that gave the "no surcharge" Visa cash advances. Turns out they also have branches in Phnom Penh and Siem Reap as well so that would be a very good option for getting cash in any of those cities. They only accept Visa, not Master Card but when I got my statement, it was in fact surcharge free as advertised. No surprises. I have a MBNA Quantum card that does not charge a cash advance fee either so the transaction cost me nothing but the interest for a week or so until I paid it off. If I was really on the ball I could have overpaid my Visa card before leaving and then just drawn down the positive balance to avoid interest as well and I would have a ready supply of surcharge, fee, and interest free cash in Cambodia. As it was the interest I paid in two weeks was a fairly trivial amount.
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There are ATM's in Cambodia. There are several in Siem Reap and Phnom Penh that dispense U.S. Dollars. ANZ Bank has several ATM's that are open 24 hours (have guards outside). They take Plus and Cirrus ATM Cards. The only fees charged are the fees that your home bank charges you.

I saw the ATM's the last two trips but I still have yet to be able to get one one of them damn things to give me any money. I assume they do give money and all and I tried Plus and Cirus cards. I even had money in my account and everything. Maybe they just don't like me.

But at least there is always Naga World Casino. They can get you a cash advance off your credit or debit card 24 hours a day if the ATM fails you at 2:00am.

There are tons of ATMs in PP. Canadia Bank is the only one that does not charge a fee. The rest of them charge like $5. They all dispense US dollars. Canadia Bank is located on St. 110 at the corner of Monivong and inside the Sorya Mall (the 5 story mall near Central Market). I think the limit is $1,000, but I regularly get $500 with no fees.