As long as they are the same OS platform. You can't switch between PC and Mac.
Techically, this is true, you can do this but....
Let's just get one thing clear however. You cannot give that CS5 to someone else. If you happen to have a second machine you want/need to run it on nobody is going to break your door down, but you will be violating the EULA by allowing another person to use that machine for InDesign work. The CS5.5 license replaces the CS5 license and so the two are tied together.