Hi scooter,
The iComfort models are all foam mattress and you can see some of my comments and analysis of it (except for the most recent additions to the lineup) in post #11 here. Since that time they came out with some of the Refined models which are also all foam and the newest introduction which is the iSeries which uses their Duet innersprings along with their particulate gel memory foam.
There are many parts of the “sleeping temperature” puzzle and some of the various factors are discussed in post #2 here and in post #6 here. It’s actually somewhat strange that Serta didn’t use the gel memory foam in the top layer of many of their mattresses when this is where any cooling benefits of the gel would have the greatest effect. This is probably because of the firmness of the gel memory foam however (only the firmest versions of the icomfort and the iseries have the gel foam on top). This is unlike some of the many other manufacturers who are making gel memory foam mattresses with better and more durable non particulate versions of the gel memory foam who use gel on the very top to take better advantage of it’s cooling properties.
Post #2 here has some of the better manufacturers and outlets in the NYC area and post #7 here also has a more detailed description of most of them.
Phoenix