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As for securing the plywood to the subfloor, I would not glue down the plywood if there is/was shifting in the subfloor. Instead, I would nail it down to the concrete using a charged nail gun that uses small, bullet-like charges. You can rent the gun. You need 3/4 inch plywood so the nails used to secure the wood floor will take. The shifting in the concrete occurring between the nails (or on only one or one row of the nails) will be absorbed by the plywood, which will bend enough along with the wood floor so as to absorb the shift but not harm the wood floor. The rest of the nails will still hold it securely in place, unless your subfloor shifts an inch or so, in which case you have a bigger problem than just surface flooring. Then nail the wood floor into the plywood using a hammer device designed for that purpose that inserts the nails at an angle through the grooves of the tongue-and-groove slots.
Because you are laying over concrete, the end product will still be a firm surface. To soften it up, you can lay 15 pound or 30 pound builder's felt between the subfloor and the plywood and between the plywood and the wood floor. Felt is cheap, and easy to roll out. Doing so will also help soundproof the floor from squeaks in the future and give it a deeper, more solid sound when walked upon.
You can also install "sleepers" under the plywood for added cushioning. Essentially, they serve as fake joists; for example, 2x4 pressure-treated planks laid flat on their sides spread 10-16 inches apart on center. They provide a bit of flex in the floor, as if it was installed on joists, not concrete. A friend of mine and I are trying using 1/4 inch thick sleepers using pressure-treated, 1/4 x 1 inch lattice-work wood strips, cutting them to 10-inch long strips, and then spreading them out in a checker-board pattern so as to distribute the load even more. Doing so should give a nice feel to the floor, while not taking too much space up.
As for the other question about glueing or floating Pergo or Engineered wood over the floor, it partly depends upon whether you think the shifting has stopped. If not, then glueing the floor down will give it nowhere to go when the shift occurs, and the crack you now see in the subfloor will appear in your Pergo floor. Floating the floor it will allow the shifting to occur under the floor with no effect on the Pergo/Engineered wood, but I am NOT a big fan of floated floors. I suspect that floated floors will fall out of favor once they start falling apart industry-wide. Moreover, even if they do survive, they often have an unnatural, "ticky-tack" sound when walked upon with hard-soled shoes. The special Pergo rubber (not foam) underlayment was designed to eliminate this sound, but it cost $60 a roll and adds so much to the cost of the floor as to make other methods/materials better: might as well buy plywood subfloor and nail it down. But that is admittedly just a matter of opinion.
[This message has been edited by Lawrence (edited July 14, 2001).]