Sure they are - answer tones are handled by AT&T's network and not by the cell phone.
The answer tones I purchased from AT&T and was using before I purchased the iPhone are working with the iPhone exactly as before.
Via AT&T's website, you can enter phone numbers for those numbers to hear a select answer tone or a random selection from all answer tones purchased. Random is selected for most of my contacts but I have a chosen answer tone for certain contacts to always hear.