The MacBook/MacBook Pro's killer feature (aside from the design and other minor touches) is its ability to run OS X and Windows at the same time. If you're a web designer, you might find that even more useful than the average developer.
Otherwise, you'll want to consider the following:
Lots and lots of RAM. There's hardly such a thing as too much (although if you have more than 4 GB, you'll want to run a 64-bit version of Windows). 4 GB is the absolute minimum for development nowadays.
The fastest hard drive [your] money can buy (note I didn't necessary say the biggest). Preferably an SSD, but you need to at least get a 7200 rpm drive. Ignore anything that doesn't let you upgrade from the standard, low-end notebook speed of 4200 or 5400 rpm.
A CPU with multiple cores is nice, and the Intel Core i5s and i7s have some nice power management features for notebooks, but you don't necessarily need the best processor money can buy. The raw speed of the CPU is less important than you might think. Most compiles, and certainly most software suites, are not particularly CPU-intensive. Opt for a nice, middle-of-the road CPU, speed-wise, from a quality processor family.
A large screen is very nice to be able to see more things at a time. I'm so spoiled with dual and triple monitor setups that using my notebook away from a desk is almost torture. Unless you need hypermobility, don't get anything less than a 15" screen. 17" is very nice, but starts approaching the "too big for a notebook" category. Do yourself a favor and keep a second monitor on your desk to plug into when you get home.
If at all possible, test out the keyboard before settling on a unit. It may be that I'm picker than most, but there are far too many notebook keyboards that I just can't stand to type on. But even if you're not as picky as I am, you're going to be spending a lot of time typing on this computer: you need something that you're fast, comfortable, and efficient with. (For me, this is a MacBook or ThinkPad.) Be very cautious of small, netbook-sized keyboards, especially those missing "uncommon" keys that aren't so "uncommon" for us developers.
Battery life is entirely up to you. If you spend most of your time mobile, you want a solid, long-lasting battery. Otherwise, it's not all that important.
Generally, brands are irrelevant: buy for features and price. People probably have opinions on this, but they're based more on experience than objectivity.