The issues are discussed here. Basically, MS has made it expensive to upgrade some VPN servers for Vista, probably so they can sell more software licenses. I don't know if Apple has done similar things or not. Generally, Apple is easier to do business with than MS, but every now & again one runs into difficulties. From a commercial systems viewpoint, there's a lot of good reasons not to depend on MS software for network services--they are expensive and they jerk you around. But it takes considerable expertise to find good alternatives.