That’s something that can only be answered on a company by company basis.
On the other hand, small to medium businesses would be well advised to make the switch. Most don’t have a Windows domain or all the headaches and full time support staff that requires. They have workgroups of machines, and if they have to go to Vista and Office 07, they’re going to have to retrain everyone.
If you’re going to have to retrain everyone, why not move them over to a different platform while you’re at it?
I'm going to ask for proof that most small to medium businesses don't have a Windows domain. Even the ones that are only running on a single SBS server have a Windows domain.