1:29 p.m. on February 16, 2010 (EST)
Rocklion -
Self-inflatables have indeed been out there for years. We still have our Thermarest pads from the early 1970s. And of course, air mattresses have been around since well before WWII.
There are probably a dozen or more companies making variations on the inflatable. The big pluses are that, like the "air beds" that are so popular as a "guest bed" for homes (much nicer than a cot), they are comfortable and when deflated, they take up little storage space (well, for backpacking, they roll up pretty small so you can get them into the pack or can strap them on the outside). The big negatives are that they cost a bunch (some going well above the $100 mark, particularly the super-cushy ones), weigh a lot, and they can develop leaks unless you are very careful with where you place them and how you protect them. When they do develop leaks in the field, they are a super pain to repair unless the hole is very obvious, and then it may be too big for your patch kit.
If you get out in winter and camp on snow or on glaciers, traditional air mattresses and even the new Cascade Designs Neo are pretty cold (the Neo is a huge improvement over the traditional air mattresses, but still really is not recommended by the manufacturer for snow camping). The reason is that the large air tubes for comfort allow a lot of convection, hence convective cooling - not as bad as a cot with no mattress, but still cold.
The self-inflatables were developed to get around the convection problem - they have an open-cell foam inside the shell that can be compressed to squeeze the air out (then close the valve to keep the air out) so that they are compact for backpacking. When you unroll them and open the valve, the foam springs back and sucks air in. Blow a little extra air in, close the valve, and you have a comfortable mattress. A variation on this is the down or Primaloft filled pad. These are somewhat warmer and lighter than the foam version (and more expensive!). Field repairs require finding the leak (hard for pinhole slow leaks) and patching it. If you go with an inflatable with its extra cost and weight, always carry a patch kit and learn how to use it.
You can still find open-cell foam pads, some with a nylon cover. These are fairly cheap, but open-cell foam = sponge. Get them around liquids (rain leaking into the tent, camping in the open, spill the soup), and they act like what they are - a wet sponge.
You can also go with what it sounds like you used to use, a closed-cell foam. These do not soak up water, and are very light (typically under 8 ounces, compared to 1.5-2 pounds for an inflatable), but bulky (since they do not compress like open cell foam or an inflatable). They can be very cheap ($5 at WalMart, Target, etc for the traditional "blue foam") or very expensive (for the fancier Ridgerest or Z-rest, close to the same size inflatables in price). They are pretty good insulators on snow, though you might want to double up.
For expeditions that involve camping on snow or glaciers, many of us use a combination of "blue foam" (the closed cell foam) and an inflatable. That way we get the advantage of the softer inflatable, which is on top of the closed cell foam that provides the automatic backup if the inflatable develops a leak (a flat inflatable is really cold and really miserable on the rocks and sticks under the ground cloth).
Despite my comments about leaky self-inflatables, in close to 40 years of using them, I have never had a leak. On Boy Scout backpacking trips, on the other hand, I have seen many flat inflatables ("It was my tentmate who punctured the pad, Mr. Scoutmaster, not me!"). And I have had companions on expeditions who have gotten small leaks in their pads (really hard to find and patch in sub-zero conditions).