This is one of the main reasons I purchased PI and I only have one computer that I use for processing. It's very rare to see a piece of software of this complexity and quality with such a forward thinking license.
From the FAQ page on the main PI website.
2.5 Must I buy several licenses to use PixInsight on Windows, OS X, Linux and FreeBSD?
Absolutely not. The commercial license gives you the right to use PixInsight on all supported platforms and operating systems. This currently includes FreeBSD, Linux, OS X, and Windows.
2.6 Do you impose restrictions on the number of installations per license?
Absolutely not. Our commercial users have the right to install and use PixInsight on any computer they own. The only limit is that only the licensed users have the right to use their copies of PixInsight. Other than the fact that a commercial licensee cannot share his/her copy of our software, we impose no restrictions.
Mike
Mike