In the US, you have to get the phone from whoever sells the prepaid service. You'd think you could just buy a SIM if you have a GSM phone, but you can't.
The best known is Tracphone which sells its phones at stores like K-Mart and Walmart, but they're kind of pricey. I see that Cingular, one of the big two carriers will sell you a prepaid phone for $30 and either charge you 25 cents/min or $1/day and 10 cents/min. They have a very large coverage area these rates should apply. Verizon also has a prepaid plan called Inpulse with decent rates but all the phones are expensive. Another possibility is Virgin Wireless which resells prepaid Sprint service. In the U.S. you pay for incoming as well as outgoing calls, since unlike in oz the caller doesn't pay extra to call a mobile.
For a calling card, poke around on the net and look for one with prices you like. The cards you buy on the net are almost without exception virtual, i.e., rather than sending you a plastic card, they just give you the access 800 number and your account number, so you can buy one before you go. Most cell phones include the whole country in their local calling area, so you only need the calling card for international calls, or if you happen to be near a regular phone and the calling card rate is cheaper. But don't forget the per-call surcharge of about 50 cents that they all charge if you call from a payphone.
Oh, that's much easier, since Europe is all GSM 900/1800 just like Australia. Buy yourself an unlocked phone before you go and bring it along. (I got mine on eBay.) When you get to Europe, just buy a prepaid SIM. Any SIM you buy in any European country will work all over Europe, but the rates are higher if you roam into other countries. A typical prepaid SIM costs E40 and comes with E15 of credit, so when you go to another country you'll have to figure out whether it's worth paying the cost for a new SIM to get in-country rates.
Regards,
John Levine snipped-for-privacy@iecc.com Primary Perpetrator of The Internet for Dummies, Information Superhighwayman wanna-be,