People are getting notebooks true, but in a couple of years most of them are going to be running a media hub in their house, or fileservers as they where called "back in the day".
Why would average non-technologically-oriented users want a media hub - let alone a self-built one where they micro-manage their components? HTPC's are currently - and probably forever will remain - a very niche market. Netflix and Hulu are only beginning to flex their muscles, but Netflix, at least, is already available on some consoles, and such sites are gaining content by the minute. Consoles, and even some TV's, can play media content via USB ports. Photos can be posted on Flickr and Picasaweb. And the list goes on. IMO, media streaming will rely more and more on centralized, powerful servers from big companies - not home users. Heck, even piracy will suffer a big blow once Netflix will have more and more movies, for example. And besides, for the purposes of connecting your media together, we've had specialized devices like A/V receivers for ages, and will get even more specialized, easy-to-use things for the modern age.
As far as gaming PC's go: With the exception of a couple MMO's (WoW), video games on consoles are vastly outselling their PC counter-parts, and game publishers are releasing crappy console ports (*cough* CoD:MW2 *cough*), instead of true PC games with content that PC users typically expect. And sure, there have always been console ports, but when game series like Call of Duty and GTA - which became popular because of their original PC counterparts - get rid of good features from previous releases on their Windows versions, it's a bad sign for the PC gaming market.
I'll side with those that say that the desktop market is dying. Not that we DIY computer nerds were ever a majority - and a sizable minority will surely remain for many, many years - but I bet we'll start seeing some of the companies that dealt to our specialized demographic dying out or changing their focus sooner than later. Remember when Asus was known primarily as an awesome motherboard manufacturer, and not the brand behind the netbook revolution?