Most the reasons stated for not using macs are specious, but some are real.
1. Mac is proprietary: This is a non-issue, at best and favors Macs by a small margnin. Windows tends to be worse because of Microsoft's "embrace and extend" philosophy with standards, and failure to document interfaces (e.g., mapi, office formats, etc.). Macs function at least as compatibly as Windows with Office productivity, email, web, and non-windows-specific development activity.
2. Limited selection of Mac platforms: This is a non-issue. IT limits selection anyway, the choices are sufficient for our needs.
3. Hardware costs more on MAC: not relevant, don't look at initial cost, look at TCO. We expect 2-3X the lifetime out of a Mac. Our management costs are substantially lower per unit. TCO favors Mac.
4. Intel mac won't run all mac software: non-issue in practice. We use MSFT office and browser as standard issue. However, see issue 6.
5. Mac market share small, 5.6%, limited support people: got this one backwards. 5.6% of the installed base is a huge number. We've found that skillbase of available mac admins to be superior to windows admins.
6. Mac market share small, limited vendor support: This is a real issue. It seems to be getting better. The biggest headache of all is working with Exchange server, but let's place the blame where it belongs. MSFT doesn't have open API's or documented protocols for working with exchange. With IMAP/POP3/ICAL/SMTP enabled life is workable, but stay away from native exchange protocols. Another problem is Active Directory integration. AD can be made to work but that requires working around all the ways MSFT doesn't comply with the LDAP standard.
7. Websites incompatible with mac: this is also an issue, sometimes. But, this is not an Apple-only issue -- this is more of MSFT's lockin strategy that makes it really easy to create websites that don't work with anything but IE. Given the relative security profiles of IE and Firefox, for example, I think we all should be concerned about this tendency to standards incompatibility from Redmon. In practice, we've found nothing we can't live without or work around.
Finally, I believe it is our responsibility as stewards of our enterprises' technology to vote with the purse - we need to push back on our vendors' attempts to create lockin. The best way to do this is to support a diverse ecosystem and eschew monoculture. Using Macs in IT is workable and profitable. Find a niche that can tolerate the learning experience and justify the short-term ROI, then edge out from there. And enjoy the freedom that ensues.