As a follow up, I found the following on howstuffworks.com:

Common Questions About
The Home Dry Cleaning Process

* Why don't delicate fabrics just shrink in the dryer?
First, the nylon or plastic bag protects the clothes from the harsh, dry heat generated by the dryer. Because of this, any bag with a hole in it can't be used anymore. Also, a ripped bag will allow the steam vapors to escape, ruining the entire cleaning process. The steam itself is not damaging because, as in steam ironing, its temperature is just below that needed to melt or shrink most synthetic fibers. However, you aren't supposed to use the kits to clean leather, velvet, suede or fur, because these fabrics can be damaged by steam.

* Why doesn't the bag blow up from the gas pressure?
The bag is sealed using Velcro strips that line its opening. This is not a tight seal; it leaves tiny spaces through which steam can be vented. The majority of the vapors are retained within the bag; but enough is let out to keep gas pressure from building up too high.

* What about the dirt and grime you can't see?
This is one of the major drawbacks to the home dry cleaning kits. The commercial dry cleaning process bathes your clothes in a large quantity of solvent that strips stains, especially greasy ones, from the fabric. With home dry cleaning, dirt removal is mainly limited to the pre-treatment stage, when you treat visible stains. Home dry cleaning kits don't address things like sweat or other body oils picked up into the clothing with everyday wear. Also, they do not remove lint or things like pet hair that can build up on the surface of your clothing. These kits can't really replace commercial dry cleaning; the Dryel kit notes that it is designed as a complement to professional dry cleaning.