Hello:
InfoPath 2007 has a combo box control which will allow a user to select from a list or type a new value. This feature is not supported for browser forms, however. In Infopath 2003, I have used a text box and a drop down box in tandem with conditional formatting to simulate a combo box.
The drop down would have an entry labelled "Add New". The text box would be conditionally hidden if the field bound to the drop down was not equal to "Add New". The drop down would be conditionally hidden if the field bound to it was equal to "Add New". So, upon selecting "Add New" in the drop down, the drop down would hide and the text box would display. I usually bound both controls to the same field.
Hilary Stoupa / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge