AT&T isn't very flexible when it comes to their plans rather voice or data. I got an iPhone when they first came out, but AT&T coverage area was so poor in the rural areas where I would use it the most that I ended up returning it. I put the blame on their coverage and was so strong about it that they ended up waiving the restock fee on the iPhone. I liked the iPhone a great deal, but if you can't use it then what good does it do you to have a nice phone.
Since that time AT&T has purchased my local cell service (Centennial Wireless) still pending approval. So, I may end up back with AT&T afterall, but at least with the better coverage.
I too have checked out their plans and unlike Jon I talk on my phone A LOT so I need a plan with a lot more minutes and really have no need for unlimited data. I'd rather them give me a very small data plan with a much larger voice minutes for a lower price. But then if you go with any PDA phone with AT&T they force you to do that freaking data plan. My previous cell service not only allowed for a small data plan, but larger minute plans with free incoming minutes and free incoming text. AT&T charges you for every text rather sending or receiving and every call. I am strongly considering going ahead and doing a new contract with my existing service prior to AT&T taking over as they will honor existing contracts. So, I'd continue to get free incoming, free incoming text, which includes unlimited outgoing text and picture messaging for $4.99 per month. However, that would keep me stuck with the same phone and that I don't like. Still I don't see myself doing another iPhone I think I've actually outgrown (like having the iPod Touch with my own phone) it to some degree, but you never know.