I guess I'm in the minority opinion here but I no longer reccomend any solution other than three identical speakers for the front. Specificly most of the horizontal format center speakers are an offense to my ears. Try this:
Sit right square in the center sweet spot and play a familiar soundtrack. Close your eyes and move your head slowly right and left and listen for the dips in the frequency response that Noah described above. The dialogue gets a bit muddy and less distinct because of the resulting harmonic distortion near the crossover frequency of the various drivers. You may not notice this in the center of the sweet spot but those sitting closest to you may. It is a serious issue because much of the sound and almost all dialogue comes through the center front speaker.
My solution may not be usable by everyone: I put three identical MTMs under the screen, all three are tipped upwards so the sound coincides with the ear level of the viewers, and the RF and LF are toed in slightly to widen the sweet spot for all three seats. The result is clear intelligible dialogue in all three seats in my "home theater" (which is also the living room).
I admit, this solution can't be used in a real theater room with more than one row of seating, but it works for me. By the way, all three MTMs are vertical - the frequency response issue results from drivers in a horizontal arrangement. I've listened to dozens of front speakers and none blend into the soundstage as seamlessly as three identical front speakers, or sound as good to my ears.
[This message has been edited by Gary McCoy (edited 06-06-2001).]