First, I've developed on OSX, windows, and Linux (mainly ubuntu and CentOS)... OSX is by FAR the easiest to work on as a developer. When I switched to OSX about 4 years ago after 6+ years developing on windows / linux I became VASTLY more productive and spent far less time fixing my computer and more time actually doing work. The price tag I'll never argue. They are over priced.
Also, who cares if it's a different kernel. First of all, it's certainly closer to the CentOS server you're (probably) going to deploy it on than your windows computer. More importantly, we're talking about Web Development here right? If you're doing coding that is low level enough to be effected by the kernel then you should probably be developing on a environment that is a clone of your production server. You can still use your Mac Book to interface with it via FTP or SSH... or as others have suggested you can always just load up a virtual box running the exact OS of your production server to test against.
Final word.. I'm not a mac fan boy or anything... I do use a mac for most of my development, but I also use linux and windows computers regularly. I have employees that use windows I think it's whatever feels best to the individual and to me Mac is the most comfortable environment for development I've found. To argue you can't or shouldn't develop on a Mac is just absurd.