Sure, the reasoning shouldn't be this way, but that's the reality. The majority of people viewing your website will be users on Windows machines using IE. Of course, this is slowly changing in the direction of Firefox, but mostly on the PC. In truth, you really have to make your website is accessible to your end users no matter what they're using. Mind you, the rare person still browsing the web on an old 68K Mac running System 7.5 or even a PC user running Windows 3.x on a Pentium I might not be your priority, but definitely most end users will be running either a Mac OS X or Windows 2K/XP computer running either IE, Firefox/Netscape, Mozilla/SeaMonkey/Camino, or Opera. And you might even want to make sure it displays in the open source desktop enironments properly as well.
