>So far I've been using drop down lists for user's to make choices from definition tables. The incremental search feature sure is nice.
>
>Now I have a situation where I want the user to easily add a value to both the list and underlying source table. If I use a combo, I can detect a new value and add it, but the incremental search feature, for existing values is lost.

I use drop down list style of combo almost exclusively. If there is a case where the user might want to add a value, I push the string "*OTHER" into the source array of the combo. If the user choses this value a text box pops up to allows them to input the non-standard value.