I use a laptop when out and about, and there's a docking station for it in the Norfolk hideaway. It connects via VPN to the machines in Essex (very easy to set up, provided you have broadband at both ends, and a fixed IP address at the "home" machine). That's an ideal setup for me.
With a laptop, what you buy is what you get - there's no significant upgrade route, apart from adding a bit more memory. The range of prices is amazing: you can get a very competent machine for around £500 (new), or pay up to £1,500 for not a lot more in terms of usable function (unless you're a mad keen gamer).
If I were buying a new laptop (no immediate plans), I'd be looking for a Centrino, 2GHz or faster, with 1GB of RAM and a reasonable sized hard drive. WiFi and LAN are essential, as is "separate" graphics (ie not using the main memory. I'd look for bluetooth if available.
I'm quite attracted to the IBM/Lenovo Thinkpad T-series (T60), mainly because I have a ThinkPad now and it's been excellent.