Jim does a nice review of iphone compatibility. 

iPhone Compatibility at the Largest U.S. Banks (NetBanker)

But the hands-down winner is Bank of America, the only top-20 U.S. bank with an iPhone-optimized homepage.

While the runners up probably work reasonably only because of smart design, BofA has optimised their site to recognise the iPhone and display appropriately.  This suggests they are thinking about target markets and taking that thinking into their internet channel.  Smart.