I find the suggest-as-you-type entry field in the page link wizard (editing a normal wiki page) and the set-the-parent interface incredibly useful. I think that such an entry field would also be quite useful as a data form field.

It would (just like the wizard, etc.) also allow entering the names of pages that don't exist.

Ideally, one would also be able to restrict the suggestions by category, but it would be very useful to me even without this. (I think I can manage to remember and type my category names, thank goodness!)