Michael Cherry, analyst with independent research firm Directions On Microsoft, based in Kirkland, Wash., said that the Transportation Department and other agencies will likely move to Vista incrementally over time.

Organizations will also need to spend valuable time and money doing extensive evaluations of the OS before any installation begins, which could push back adoption further, he said.

"When I read the [DOT] memo, I heard an IT department asking its people to refrain from buying ad hoc Vista licenses and machines, because they can't support that yet," said Cherry. "Something else to consider is that IT people have a lot of projects that they're already working and limited resources; Vista is not the kind of thing that merits dropping everything else to move on it quickly."

Cherry said that he does believe federal agencies will begin adopting the OS at the same rate that they retire older desktop systems, although he noted that IT departments will likely give more consideration than ever before to alternatives including open source and Apple products.

The first types of government users the analyst believes are likely to install Vista for functional reasons will be employees who carry sensitive data on their laptops, or view such information remotely on their PCs, he said. Those users will be seeking the onboard protection offered by the BitLocker encryption feature of Vista, according to the expert.