It would depend, I think. IMHO, it would be easier for the corporate mindset to go with Apple at present than it would be to go with Linux or BSD because Apple's already -there- in terms of support offerings. And you can definately move client software over to either/or since it is now possible through FS/OSS to connect to Exchange.

As a follow-up, I offer an additional link: Of Course Macs are more Expensive