I have 3/4" plywood subflooring throughout the house. However, it appears that in portions of the house, the subfloor is somewhat uneven.

Instead of using shingles or felt paper to even out the floor, what if I were to screw down 1/8" lauan as a top layer for the entire area and then nail through that? Or, are there issues involved in doing that?

It is too flimsy and will contour to what is there now. You want to use no less then ½" plywood, and even then, your going to need to sand down the high spots and fill the low spots, or that second layer is going to flex, Flexing is squeaking.

Thanks. I had a feeling that's what I was going to hear. No quick fixes it seems.

Well then, as I've learned from these forums, I'm going to screw down all the squeeks prior to starting and then use the shingles and/or 15/30# felt paper solution for evening out the subfloor for nailing.