Well I found these two faq's on MS's knowledge base.
http://support.microsoft.com/kb/927007
Q4: How many times can I activate the product?
A4: There is no limit to the number of times that you can activate the software product on the same computer. For example, you can remove and reinstall or reactivate the product any number of times. An additional activation is provided so that you can remove and re-install the product on the same computer or on a different computer. See the Microsoft Software License Terms for more information. The Microsoft Software License Terms are located in the game folder.
Q6: Can I install the product on more than one computer?
A6: You can install and use one copy of the software product on the licensed device. The licensed device is the device for which you have accepted the license agreement. You can remove the product and then install the product on another computer. However, you cannot share the license between two or more computers. See the End User License Agreement (EULA), for more information.*******************
That being said I have FSX installed on 2 different machines at this moment. And on the second machine I have it installed twice, once on each partition (vista/xp) I have upgraded computers since the release like most people so the above seems not accurate. I think the part about installing unlimited on the same machine might be right though.