For travel (but not for main workhorse) definitely 12", if you're a backpacker type. If travel means driving around in car, then it doesn't really matter.
For what it's worth, I vote for Thinkpad too. For travel definitely an X-series (they lack the optical drive though). I myself own X60s. I'm sucker for light laptops since when I carry one, I carry all day in my backpack and I go around on foot or bike, I don't drive. While you may not notice the difference between 2,3 and 2 kg, you will definitely notice a difference between 1,4 and 2 kg when it's you who has to lug it around, not your car. My laptop with power brick is lighter to carry around than my K100D + external flash.
I get to regularily test laptops and while I may be partial to Thinkpads (have owned/used about 5 of them), I've seen just one or two other lappies, that I'd even start to consider using instead of my current one. For example HP 12" NC2400 is also nice no-nonsense work laptop that I'd buy blindly, should I need to. But people have different needs, and have different areas of pickiness.
Whichever brand you choose, when you look for travel laptop, consider following points before deciding. These may or may not apply to you, depending on what type of traveller you are.
1. Does the lid close completely and tightly? When travelling, all sorts of things make it into your bag and when laptop case doesn't seal off the keyboard you'll end up with stuff getting in between screen and keyboard. And that IS NOT nice. Thinkpads have a lid that protects the space between keyboard and mouse from all sides, some other brands have more or less similar protection, check it by looking at closed lid from each side.
2. Can you place the laptop on each of the four sides or is one side more vulnerable than others? It's not a problem when you use dedicated carrying case, but when not, it may be a problem, when in order to pack effectively you need to place laptop not horizontally but vertically. Or vice versa. Or upside down. Can your laptop handle that or are there any bumps which take it out of balance, or any fragile thingies that can break off or give in?
3. Can you lift it or open it with one hand only? I usually have time to carry my laptop with care at home or at work, but when I'm on the road - internet cafe, a bus, an airport etc - I may only have one free hand to lift it or carry it, while other is holding bags/coffe/tickets/etc. Is the case too slippery? Is the weight distiribution good for you? Can you open it with one hand only?
4. Can you use it comfortably on your lap as opposed to using it on desk? Some laptops have really bad weight distribution with open lid and tend to fall back or forth, when you're using them at an angle (on your knees, with feet put up or curled under your).
5. Does it have neccessary number of ports/slots? Does it have a memory card reader you need (SD-slot, for example)?
6. Can you write on it using it on your lap? Some models are just plain uncomfortable for that, be sure to try it out. Can you use the pointing device while using the laptop on your lap? For most people touchpad seems to be more convenient when working on desk, but trackpoint, when working in confined spaces or on lap.
7. Is the screen readable under various conditions you might encounter? Reflective screens are great when you can control the lighting, but suck, when you have glaring lights from behind and sides.
8. Does it have ventilation holes on the side or underside? If hot air is blown out from underside it's usually not a problem when laptop is on hard surface (desk), but is plain suicidal, when you use on soft surfaces like blanket, lap, hotel bed, on top of your bag etc.
9. Does it have unconditional international warranty? What do you need have/prove when you go into repair abroad? Is the model number enough or do you need any other certificates or proofs of ownership? Are warranty conditions different for different problems (broken case corner treated differently than not working cd-drive?). It may vary wildly between manufacturers. Thinkpads, for example, have unconditional warranty, things under warranty will be replaced/repaired with no questions asked. Some manufacturers are known to point a finger on you first and repair later. While you may have time to argue with them when you're at home, you don't want that hassle and may not have the time while on road. Warranty conditions also vary within manufacturers different models, the line typically being drawn between consumer and business models. Find out before committing.
There are probably other things you should consider, but these are first from top of my head, based on my own experience. Your mileage may vary.
Last edited by aabram; 04-03-2007 at 10:50 PM.