My 0.02$ as I've been using Windows and Apple machines for a long time now. Right now, my computer of choice is the top-notch 15' MacBook Pro. I bought it in June 07 and it followed me on several flights this year. Before I decided to buy a MBP, I had a second generation MacBook and before that I used an Asus notebook and standard Windows computers for over 10 years.
First of all: Apple is not the optimal choice for everybody!
I'm not an Apple fanboy, I like working on OS X though because I don't need any specific programs that only work on Windows. If there were any, I'd probably be using an IBM/Lenovo or HP notebook. It totally depends on your needs if an Apple notebook is suited for you but in general it can be said that if you're not dependent on any Windows applications, OS X will serve you better. OS X runs more stable, you can really rely on your machine to work when you need it and you don't have to format once in a while to clean out the registry. This means you save a lot of time just by not having to tweak/clean your OS.
The MacBook Air
Imho, the MBA is really good if you don't need a DVD drive everywhere you go. On my travels, the MBP was still better than any other notebook I took with me because it was thinner and lighter ... but the MBA gives me the power of an MB/MBP(without the graphics card but hey, I won't be designing in Cinema4D abroad) everywhere I go without taking up too much space or weighing too much.
Especially if you already own an iPod or iPhone, working on an Apple makes life more easy. Apple products just work perfectly together.
The MBA is designed for people who want a good machine they can take along so this would be the ultimate computer for travelers like us if we're not dependent on Windows applications. And even if one would need some Windows app, there would still be the possibility of just installing Windows XP through Boot Camp. I can say that even after 10+ years working on Windows, there is NO application I miss on my Mac and I don't have Boot Camp installed.
Personally, I would get an MBA if I hadn't just bought a MBP because traveling would be much more convenient.