From the requirements you've outlined, I'd choose a polycarbonate (white) macbook (they've just been updated too). they're quite rugged, have a good screen, easily upgraded for the RAM & HDD (4GB RAM and a 500GB drive drops straight in) and DIY upgrades don't invalidate the warranty. DVD writer (superdrive) is already built in, perfect for burning discs to mail home. The PSU is pretty compact, and the plug piece is interchangeable for different countries too. Dual boot to windows is fully supported by Apple and MS too
You also get the benefit of getting excellent support in the Apple stores & franchises worldwide. I've taken advantage of this in the US and the far east. You simply don't get this with any other supplier.
Warranties are global, and can be extended for a reasonable price, as long as you buy away from the apple store. Have a look in Cowasaki's thread for details. This adds phone support too.
If you can get an educational discount, student or teacher, you'll save a chunk on a new Mac too (may get you a new one for 2nd hand prices)
Also, keep an eye on the refurb store. Same kit, full warranty, just no fancy box
http://store.apple.com/uk/browse/home/specialdeals/mac?mco=MTE2ODI
Netbooks are great, and ideal as a second machine, but I wouldn't want one to be my main device for 2 years whilst hacking around the globe. I'd want something that I know I can get global support on, and is fairly rugged (since you don't list a ToughBook I'd opt for the MacBook).
I'd also be less inclined to go for the aluminium bodied macbook as the screen is glass, and may not stand up to the rigours of cycling.