The ReactOS people have thought about these issues, and seem to have a pretty good handle on it:http://www.reactos.org/xhtml/en/dev_legalreview.html
Clearly, there is enough amibiguity that if MS decided to challenge the project, it would almost certainly fold under the weight (both time and money) of fighting the challeges.
My guess is that MS is not concerned, since Windows has a head start of many years, and the project poses little real commercial threat. Going after the project would just waste MS resources and cause bad publicity with no real upside.
However, I'm sure that would quickly change if MS found evidence that Windows source was misappropriated...