For what it's worth, My MacBook is about 14 months old, I got it shortly after they released the second Gen C2D macbooks. It's the faster of the CPU Speeds as well. With that, I've got 353 cycles on the current battery and I can't tell any difference in run-time. With the screen moderately dimmed like I have it when I'm indoors and surfing the web it still runs for more than 4 hours. Play some tunes or kick the brightness up to 3/4 or so and that gets cut down to a notch under three hours. Running full tilt with full brightness and heavy CPU Usage (like iTunes visualizations at 60fps or Windows 2000 running in Parallels so I can VPN into work) it still runs for close to an hour and a half. I'm betting the MBA battery will last you quite a while before you need to go to the Apple Store with your tail (and pocketbook) between your legs.

As far as riding with it on your bike, a good padded cell does wonders. On my road bike, I just throw it in my panniers. When I switch to my Mountain bike, there are a few considerations for my situation:

1) I switch because it's snowy/icy season and my road bike doesn't handle slippery surfaces well. Still on a MTB I'm prone to fall over, so I choose a backpack with my MTB. I don't want to break my bike's fall with a MacBook.

2) Since my MTB isn't up to the 30 mile round trip (or rather, I don't want to ride a slow, heavy bike that far every day), I use the bus to shorten my ride to a more manageable 5-10 mile RT. Wearing a backpack for less than 10 miles a day isn't too bad.

Now, I've had two really good tumbles with my MacBook in the backpack. One of them broke my face and a bunch of teeth, but not the MacBook. This is a padded laptop-specific backpack, though. I've also had one really good tumble on my road bike with the laptop in a pannier, protected by a padded cell.

I also had the same padded cell fall off the back of my hybrid's rack at about 18 MPH. I had it bungeed to a rack back when I first started commuting, bad idea. This wasn't my MB; it was carrying the G3 PowerBook I had before I got this MacBook. The G3 Wallstreet, as well, survived just fine. A quality padded cell in a pannier will ease your mind and get the weight down low and off of your back.