The time has come where the PC plays more of a role in life than just being something that sits in a room and is used from time to time to do work.
Increasingly I've found it would be useful to be able to pick up a laptop and move to a different room. Wireless networking, better battery life and smaller notebooks make this something to consider.
There is also the need to be able to work in several locations, be it in different homes, places of work or just travelling between places. Using a laptop saves time in not having to duplicate installs of applications and then just carry the data around with you.
As prices fall and as people let the PC become more a day-to-day tool rather than something for doing work with then it has changed peoples perception and needs. Better laptops in terms of small size/faster processor etc have meant they are viable alternitives.