In Cambodia there are two official currencies:
– USD $ US Dollars
– KHR Cambodia Riels

Current rate exchange is, on the 28th of December 2010:
1 USD = 4050 KHR (so 4000)

ATM machines give both currencies. Notes dispensed are: 100, 20, 10 $; 10000 KHR.
The problem is that there is no an specific option to choose the currency, it depends the amount you withdraw you receive a currency or the other one.

Prices can be in both currencies or just in one. Obviously more touristic is the place, more chance of having prices in $.

Most of the times change will be given in Riels, over all because there are not USD cents in Cambodia but prices show them! Try to pay exact amount if possible.
Cambodian people are usually honest. Anyhow, check always the change.

Checking the change might be a nightmare if you think in EUR.
Some little help:
A. If you pay in Dollar.
A1. Convert in Riels the amount in $ you pay and calculate how many Riels you must receive.
A2. After receiving the change convert first the $ received (if any) and add the Riels.
B. You pay in Riels.
B1. Convert the price in Dollars to Riels and pay. 0,25$ are 1000 Riels.

Try to have some amount of both currencies always.
It’s a bit annoying, but not difficult.