yes the iPhone requires an iPhone plan, you can buy the phone contract free, but the phone is locked to AT&T... The only reason for doing this is let's say you were planning on moving out of the country or something in a year b4 your 2 year contract ended...
The deposit is not for the phone it is for the service, AT&T does not have a pay as you go plan anymore... They are not that strict on credit call and ask... or goto Best Buy or goto
Apple - iPhone - Buy iPhone 3G that process should tell you the credit