I had a custom field type in my MOSS solution that was a simple CODE type.  This was based on a Text type but all it did was to uppercase any value.

However, when I referenced a list that used this field type from InfoPath the column would not display in InfoPath.

I haven't figured out how to do this yet so I reverted back to a Text field for now.  I'm guessing there is an attribute I need to add the the module.xml or maybe in code? If you have a solution, I'd love to know :)