I switched to a laptop for a sole computer almost a year and a half ago. This also coincided with me getting an XBox.
My rig wasn't the fastest, but at the time it was pretty pimp. A 2.4C P4 @ 3.2, 512 DDR400, Radeon 9500 Pro, etc. My laptop is a far cry from it. It's a HP Pavilion ze1210, Athlon 1400+ (1.2ghz), 512MB SDRAM, 16MB S3 TwisterK. But the thing is, I can never imagine going back to a desktop.
I used to be a hardcore PC gamer, and upgraded my video card to the latest and greatest mid-range every two generations or so, and then I got an XBox with Live!, and it is likely I will never buy another gaming desktop PC again. I can do just as much gaming with my XBox on the comfort of my own couch without having to dump money into upgrading my PC.
What I did was take a Compaq iPaq (a SFF desktop) which consists of a P3-1000, 512MB SDRAM, and my 160GB HD and I have it next to the TV where the router is, with VNC installed on it, and I use it for all my torrents and stuff. It's like a little file server. I've decided to upgrade it to a Athlon XP with XP MCE in anticipation of getting an X360 with the Media Center Extender built in, and it also serves as the print server. My wife also has her own laptop (a swank Dell 600m), and there are three other desktops in the house, but they are for my inlaws. When we move out, it will be just two laptops and the file/mce server.