Why use a platform independent technology to deliver your application to customers and then tie your implementation exclusively to a single platform? This is exactly what Siebel has done with their CRM OnDemand product.
Despite having built the application to run in a web browser, the technology that should allow vendors to break their dependence on Microsoft, the Siebel folks have tied their implementation to an MS Windows specific implementation of Internet Explorer. That's right, even using Microsoft's version of Internet Explorer for Mac OS X you cannot run Siebel's CRM OnDemand application.
And forget about any non-IE browsers.
Understandably, 95% of the world uses Microsoft, so vendors should support their Windows customers. But it isn't that hard to make a browser based application work on non-Windows platforms. And if you don't care about supporting non-Windows customers, why use a browser based application anyway? A much better user experience is available by building a native Windows application.
Siebel is not alone. I have found that more and more web sites that I visit are Mac unfriendly. Recently I upgraded to IE 5.2.3 (the most recent release for Mac) and discovered that Macromedia's Flash Player has stopped working. Whether deliberate or not, Microsoft is succeeding in balkanizing the Internet -- diverging their browser (even on their OS) from the marketplace. Given the economics, most companies will chose to invest in Windows specific solutions. The Microsoft monopoly wins again.