Yes, Pro Logic II will produce five channels of audio. However, as Sanjay points out, that is going to be dependent on the source material and influenced by the Center Width setting. A mono source (even if carried equally by both left and right channels) would tend to be reproduced solely in the center. Similarly, I'd expect programs that are nothing but dialog to tend to do the same thing (since the dialog is going to want to be anchored to the screen and therefore should be steered to the center).
Here's something that might be useful to our discussion. The quote below was taken from page 31 of the 990's manual.
Center Width blends sound from the center channel into the left and right channels to create a more seamless front soundstage. At a setting of STEP 0, no sound is mixed from center into left and right. At the other extreme, STEP 7, this control can completely deactivate the center speaker so a phantom center image is created from just the left and right fron speakers. STEP 3 is a good starting point for many systems and types of material.
Based on that, STEP 0 on the 990
should yield no data in the left and right channels.