I just bought my first ever laptop (see sig for details) and cannot see any reason for me to want to upgrade it for a fair few years... I'm not a heavy gamer (although I did specifically buy this one to play Civilization IV !) and so do not need to keep up with the latest bleeding edge technology.
In the past, I have tended to have a PC for 6-8 years with a midlife upgrade of a few things to keep it functional. I suspect that the midlife upgrade will be impractical with my lappy so I got it maxed out on memory and the minimum CPU... I'll pinmod a 1.6 in a year or two if more CPU power is needed.
In 4-5 years, if I need a replacement, it'll probably be one of the convertible ones with the screen that flips over and will have 4 or more 64 bit cores with flash memory rather than a hard disk... It'll have a propane based fuel cell battery that lasts for 12 hours and can be recharged like a cigarette lighter.
It will also be the size of a paper-back book with 300 dpi resolution, so I can carry it in my pocket.
It will only work if it is within 3 feet of me, since I will have a smart RFI chip implanted in me that will verify my identity.
If I wish to, I can leave it in my pocket and play games using the bluetooth X glasses, earbuds and glove and will also function as a cellphone, camera, GPS system, wallet and personal On-Star service. It will not make the coffee but can arrange for one to be freshly prepared and waiting for me to grab as I drive past the nearest Starbucks, if I want.
The one thing I don't think will quite work will be the voice recognition that will allow me to dictate documents, program code and emails using whispers or silent speech.