Actually, the ability to use purchased music (and maybe even video) as ringtones was one of the reasons I was going to buy an iPhone.
When I found out that this particular feature wasn't available (at least not until a major software revision) I opted out of buying one.
Better ringtones could help too - the ones I experienced in the store seemed too quiet.
But, now that I think about it. A lot of folks have a lot more media then what could possibly fit onto an 4g/8g iPhones flash. So if you go and assign a bunch of different songs as ringtones for specific (or nonspecific) callers and then change out your entire iPhone library, you're left with a bunch of unassigned ringtones for callers.
I suppose in that situation, it could just go to a default ring. Hmm.
Either way, my deal breaker is songs as ringtones.