One of the biggest problems with being a web developer is being able to make sure that all your web site is cross platform. So that it will work on all browsers.
The majority of browsers (firefox, opera, etc) are cross platform. so no matter which os you run you can run these browsers. The problems come with IE and Safari which are only available in Windows, and OS X, and generally most developers have access to a windows machine since IE is the biggest bastard of a browser to develop for, and and the other side Safari is pretty standards compliant.
So most people will take route of getting a Windows machine to sort out IE, and hope more than anything that it will just work.
Now with the release of the Intel Macs 2 things have happened. You can now dual boot, or use parallels on your Mac to test IE, or the old PowerPC macs have become dirt cheap and will run OS X 10.4
Because I just wanted to dip my feet in the Mac waters, I picked up a PowerMac G4 400 for a couple of hundred bucks, and I get to play with OS X, and see if it is for me, and I can test Safari without any issues. It is a bit slow, but to play MP3's and check the layout of pages in Safari it is just right.
At this stage my next laptop is going to be a Macbook, and I am very happy with this.