Are there compatibility problems with buying iPhone 4 overseas? I have found unlocked iPhones 4 from USA and malaysia to be so much more cheaper than Australia, are there any compatibility problems associated with this? 
I heard if you buy an iPhone in US or other countries and take it back to Australia, itunes will only accept credit cards from the country of manufacture when purchasing apps i.e.  if bought in US, must use US credit card. Is this true?
As long as the iPhone is unlocked and to connected to any network, you can use it in any country. 

You will need to check that if you are buying in the USA and you are getting an iPhone 4 you will need to buy a GSM phone as all Australian networks use GSM after Telstra shut off CDMA, if you are buying an iPhone 4S you need't worry as these combine both networks for maximum coverage.

You may find it useful that some iPhones in some countries are always sold unlocked such as France and Belgium where it is against the law to sell subsidies handsets on contracts.