Pete,
There was no difference in my case between IGeoFeatureLayer.FeatureClass and DisplayFeatureClass properties-- both returned the correct feature count that I was expecting.
I did have to create a feature cursor and loop through the features and union the envelopes as you indicated, so that was the solution. I just now need to go back and work in the case when a group layer is selected and have to drill down to each of the feature layers.
Thanks again for the quick response!
-- Glenn