[robertdempsey]

If you look purely at what is needed to build a web app - the programming language you'll use, an editor and a web browser - and you're a lone developer (not working with a team with existing tools that you too will need), it's hard for me to come up with a solid argument for buying a Mac purely for that (though I'm a huge Apple fan). You can also use apps like Gimp on Linux to create graphics.

Just about all business-ey apps you might need can be gotten for free from Google Apps if you aren't worried about the big G having all your information.

If you already have a Mac though...

I also agree with the points of other commenters here. If you're working on the same OS you're deploying to and have the exact same software installed changes of FUBAR'ing the server by accident are reduced.