Not an option, as AT&T no longer offers the plans. Because of this, there is no way to get a telephone rep to switch you. The only work around is by downgrading to an iPhone 2G (original). The only data plan for those phones are unlimited. Once on this plan, you can then "upgrade" to a newer iPhone and be grandfathered in on the newly acquired unlimited plan. This is what I did, and it works like a champ.
Since Verizon has expressed interest in teired plans as well, I don't see them offering unlimited past the iPhone4. In my opinion, the only reason they are offering unlimited on the iPhone4 is to drawl as much competition away from AT&T as possible. However, once AT&T customers have their contracts expire, Verizon is expecting a mass exdus over to their network. In my opinion, this is when Verizon will adopt the teired data plans (Summer of 2011 or 2012).
After that happens, don't expect unlimited plans to ever come back! Just pray lots of Wi-Fi hotspots start popping up.
Cosmo