Actually, I had a prepaid dumb phone and an iPod touch. I finally caved into the iPhone 4S because it was one device instead two or three (the third being a digital P&S camera since the iPod touch's camera is pretty mediocre).
Things were particularly groovy until about a month ago, when AT&T changed their prepaid GoPhone plans to eliminate data packages from their daily plans.
I was using their $0.10/min. plan with a data package that would roll over if I bought a new data package before the month's end. In the end, I averaged about $12-13/month in cellular charges, mostly cellular data since I got so few calls and texts were handled by the Google Voice app.
That was awesome, far better than juggling 2-3 devices and still getting HSPA+ cellular data performance when I was out of WiFi range (I have great WiFi both at home and work).
But AT&T wised up and the GoPhone plans are no longer a competitive option for iPhone owners.
I still have my Motorola dumb phone as a backup device. It's an unlocked quad-band GSM phone, which means I can take it practically anywhere in the world and slip in a SIM and get local service. Right now it has one of my old GoPhone SIM cards, but I also have a Tru SIM.
Having done the dumbphone/iPod touch combo and now the iPhone, they are not the same user experience. I'm doubtful that I could go back even though I would be saving money. It's all the little intangibles that add up. For example: I loathed texting on my dumb phone, but it's easy on my iPod touch/iPhone. The camera on the iPhone 4S kicks ass. Always on Internet plus GPS on the iPhone makes for one heck of a great navigation device when you're traveling, whether you use free Waze or payware apps like Garmin or CoPilot.
I know where I can get free WiFi in my town, I don't know when I'm traveling.