Battery life is crucial, but there's an equally important test for any notebook that's going to get regularly used on a plane: is it small enough to fit on the tray, allow typing when there's someone sitting next to you and not end up half-closed when the person in front reclines their seat?
In the course of the Hand Luggage Only trip, I've noticed plenty of people making use of PCs in the air, but a surprising number fail these criteria, resulting in much wailing and gnashing of teeth. The most common problem is picking something that's too large: a 17 inch screen might look lovely in the office, but it's just not going to cut it in a standard economy seat, especially if the passenger in front is taking a nap.
Having a smaller machine (and the Portégé R600 I'm taking on this trip definitely fits in that category) also has more subtle benefits. Not only are you protected from reclining dramas, but there's enough space to actually fit the drinks and snacks next to it. That's obviously less important if you're on a pay-for-the-food budget airline, but still useful. A smaller display can also translate into longer battery life, depending on what you're doing.
So far, I've successfully used the R600 on planes ranging in size from a Dash 8 through to a 767. Apologies for the rubbish photo, which was taken on a 717: it's near impossible to take a photograph of yourself using your own laptop in a confined space.
Once I've returned from Perth on a 747, I'll have tried out pretty much the entire Qantas domestic range. Of course, it helps that I never work with a mouse. Several commentershave pointed out that wireless mice aren't an option in the air due to radio transmission requirements -- if you simply must have a mouse, grab a cable and pray that the next seat is empty. (Again, a smaller notebook will give you a small pool of mouse room, though probably not enough for Photoshop.)
Throughout May 2009, Lifehacker Australia editor Angus Kidman will be travelling throughout Australia with just one carry-on bag for the Hand Luggage Only project.