I have ReSharper installed with VS on a machine. I want to now use VS on a different machine, with ReSharper. Is this easy to do?
Yes. You need to have the "Personal License" in order to do this in compliance with the license terms. Simply install Resharper, and input your name and registration key.
According to: http://www.jetbrains.com/resharper/buy/buy.jsp#personal_licenseTerms
A Personal License is a named-user license which restricts the use of the software to the person who purchased that license. The license allows for installation of the software on different computers (at work and/or at home), provided that multiple instances of the software will not be used at the same time.
If you have purchased another license than the Personal one, I would suggest you contact Jetbrains and ask them if your license allows it. In particular, I think that whether you can do this with the Commercial License, would depend on the exact usage scenario.
I personally have a Personal license and use ReSharper on both my work pc, home pc, and on my laptop - with no problems. You can only use it on one of the installed computers at the same time.