I bought office 2007 Professional. The paper in the container seems to indicate that the Home/student version can be used non commercially on 3 computers. However, nothing is said about the other versions. That implies that the PRO version will install an unlimited number of times, but that is not explicityly stated.
Since we cannot trust MS to tell us if this is so, there is one way to find out. I currently have mine installed on 3 PCs in my home and office. I will likely try others and see what happens. I can always call MS and retire a copy from one PC if necessary.