I'm no big fan of Windows, but it's just too much of a migration to ask enterprise customers to drop it. The vast amount of installed software that's dependent on Windows pretty much negates the chance of any big shifts in the business community.
However, moving their web-facing apps away from Outlook and Internet Explorer is a more achievable goal. It's just two apps in the pile. You could use security and "brand-ability" as key selling points.
If the business community went to a diverse range of web-facing apps, then security overall would improve. You would see the end of the "fat target" of a browser or e-mail client with 80% market share. Each different browser and e-mail client has it's own set of vulnerabilities, but they need to be exploited individually. This would reduce and fragment the impact of any malware attack.
And don't underestimate branding. Many browsers and e-mail clients support client created "skins" - just think of the feel-good aspect of that to an exec. Each company would be able to brand their browser and e-mail client with their own logos and corporate color scheme. Senior execs love that kind of stuff. They know that the first step in establishing strong brand recognition, is embedding that recognition within their own staff.
Down the raod, you could do that with highly skinned and customized versions of Linux, but right now, it's just far too much to ask for that big of a leap. The first step is to start moving them, application by application, to programs that are platform independent.
Just thinking out loud. Actually, I've been thinking this for a while, just haven't much spoken about it.