I've been thru several notebooks and these are some of my thoughts (humble thoughts!):
Any notebook will do considering that you can get the fastest processor you can, along with lot of ram (1 gig minimum for me), a good quality sound card (I use the echo indigo IO, addresses the audio latency issue, processor included)), and have a fast hard drive (7200 would be great, the main reason is that if you are using large samples, or streaming greater amounts of audio, you should probably have a faster HD).
I also found that IRQ configurations have always seemed to be more favorable with the AMD systems. Every Pentium 4 or Centrino system I have checked out seemed to have the video and audio on the same IRQ, but that wasn't the case with the AMD's BUy in real world applications, it hasn't stopped me from using my Dell 9200 Centrino ( 1.6), and I'm currently doing trance, and use a higher amount of active plug-ins. I personally consider this a more jild issue.
Then there's the screen. You can also hook up an external monitor, but there are times when you may not want the extra bulk. My HP had a 15" screen and served me well for a few years, but when I went to the 17" Dell, it was night and day. The bigger brighter screen makes everything look great and you can put more things on there, instead of having to layer everything. It really makes a difference.
I used Reason for a while back then, and it ran very well on my 1.8ghz AMD XPM with 1 gig of ram, 5400 HD. Cubase SE and SX ran well too. My current rig, the Dell 9200 1.6 centrino runs better than the HP, 1 gig ram, 5400 hd, running Cubase SE.
I have spent many hours configuring these notebooks, but basically its been BlackVipers stuff and the stuff on www.musicxp.com
If you are looking at the PC route, there are many choices, but just make sure you get:
1) Fastest processor you can afford
2) As much ram as you can afford
3) fastest HD you can afford
4) good quality soundcard. (the native built in cards are almost always noisy, with the ezception of the apples, they seem to be excellent, but my HP and old Sony s360, were noisy. Incidently, the Sony, with a 4200 HD, couldn't run the Halion or Kompakt samplers at all. The drive was just too slow.