I have implemented URL Hacking for Opportunity Object on New Opportunity creation from Account record(to pre-populate 'Opportunity Name' standard field with 'Account Name'). We actually have 3 Record Types on Account and URL hack works for only one Record Type, for other 2 Record Types, it's not pre-populating 'Opportunity Name'.(opp4 is working, opp3 is not)
/006/e?retURL=/{!Account.Id}&opp3={!Account.Name}&opp4={!Account.Name}