ATMs can be found in Phnom Penh, Siem Reap, Battambang, Sihanoukville, and Kampot; both debit card withdrawls (Maestro, Cirrus, Plus, VISA) and cash advances on credit cards are possible. For the rest of the country it's best to stick to cash or traveller's checks (in US$).
VISA and JCB are the most widely accepted credit cards; MasterCard and American Express cards are slowly becoming more widely accepted.
Note that ATMs will dispense US$ in varying denominations. If you receive bills in poor condition(especially $50 or $100) from an ATM attached directly to a bank try to change it there immediately as it may be difficult to change later.
Please note that ATMs throughout most of Asia only accept a 4-digit PIN. If your PIN is more than 4 digits, best to take care of that at home before you need cash and find yourself out of luck.