Those costs, DeGroot noted, don't include the $100 to $165 additional per-PC fee that non-Software Assurance subscribers would have to pay.

On the argument that companies that skip Vista could be stuck if independent software vendors (ISVs) stop supporting their XP apps before they release a Windows 7 version, Fulton County's Terrell agrees that is a legitimate concern.

But DeGroot remains unconvinced, saying he hasn't heard of any ISVs, including Microsoft, making such an announcement yet.

"I don't think that Microsoft has announced plans to pull Office support for XP," he said, "and Office 14, as far as we know, will run on XP, so there's a notable example that the rest of the industry is likely to follow."

DeGroot's conclusion is that most companies will do well either holding out for Windows 7 or moving to Vista only as new PCs are purchased.