iPhone SDK 3.2 Beta 4 (I am downloading B5 to see if this clears this up)
To me this seems like a bug, but maybe I am missing something. The linked test program illustrates this problem.
I have several UITextFields that I am linking together using the Tag property. In textFieldShouldReturn I get the next UITextField and set it as the active responder. The problem arises with the IB settings for the keyboard type. In the test program I have the first two text fields set to default. The next three text fields are set to numeric and symbols. As you move into the set (bottom three) that use the numeric/symbol keyboard the keyboard will toggle between numeric/symbol and the default. What I mean is the third field will display the numeric/symbol. The second Default. The third numeric/symbol. If you add more text fields setup as numeric/symbol this will continue on.
Now if you tab into the forth field (it should bring up the default) and then manually select the numeric/symbol. The problem will disappear. I have tried various things to track down why the problem disappears. No luck so far.