I think that while PET bottles may gain a foothold as fermentation vessels (I use Better Bottles, even for my mead, and it looks like other people are starting to also), but I don't think that plastic bottled beers will be sold commercially (large scale) for a long time.
There is way to much of a stigma associated with plastic (it even affects aluminum to some degree). People just don't expect good craft beers to be bottled in anything but glass.
I think that the effect is most noticable in the wine industry, as it is exceptionally slow to make any changes. No matter how great the wine is, if it comes in a can, or a plastic bottle, people don't want it. Even if it is cheaper. The same goes for artificial corks (which, after over a decade are still not widely adopted, despite the advantages), and screw tops. Many of these innovations actually perform better at storing the wine than the glass bottles do. Look at wine in a box for a good example. The wine is kept in an oxygen-barrier bag with a spout on it. As it is dispensed, the bag collapses so that the wine never comes in contact with oxygen. The wine can then keep longer without oxidizing and showing off flavors.
Plastic bottles would seem to be a great thing to breweries though, they would be cheaper, easier to handle (no breaking) and much easier to make. For the homebrewer, the bottles have all of those advantages, plus they are easier to clean/sanitize and easier to cap. The only thing that keeps this stuff from being adopted is the stigma that plastic has in the consumer's mind.
Personally, I like the weight and feel of a glass bottle in my hand. Of course, my brews are bottle conditioned and I can't drink directly from the bottle, so I suppose I wouldn't notice anything if I switched to glass.
