I've architected, designed, and developed ecommerce engines AND interfaces for fortune 500 companies, so I think I'm fairly qualified on this topic.

First off it is NOT as easy as just implementing a "css xhtml" website. Full specs are ONLY fully supported within the most recent browsers like Moz. If you refer to the various statistics across agencies most people run either IE 5 or IE 6, and most here is a number over 90%. I can name multiple failure points in just about every one of teh browsers you mention... why don't you try running Opera 7 on a system with type manager installed and watch what happens.

I get so sick of hearing people who whack a couple sites together for limited consumption lecture about how to develop and implement systems that need to support hundreds of thousands of users. CSS/XHTML using pixel positioning falls apart on any browser that doesn't have spec support, and to any business that is unacceptable. Until ALL browsers but say less than 1% support the specs fully it is neither acceptable nor feasible for a multi-million dollar company to risk alienating users with a site that doesn't work properly on ALL systems. At this point that is still a site that uses html 4 transitional.

Whether anyone likes to admit it or not most people are still using computers they bought two years or more ago, and most of those machines are running IE 5 or 6 and nothing else. The general public neither cares nor wants browser options, they just want it to work. MS puts it in front of them, and until computer manufacturers CHOOSE to install non-MS browsers in their machines pre-delivery you won't see many changes.

As for Flash it really has no place as a page element outside of illustration or product zoom/review capacities. Using any linear scripted interface for navigation and core page elements is a mistake, and general is only done by folks who either don't have large user groups or don't know any better. Of course Flash MX is a little better in its support for database access, etc., but I still stay away from it.

-rt