I successfully installed MatLab 2011a on my MacBook Pro running Mac OS X 10.6.7.
However, I also have an old installation of Matlab 2010b on the computer, how do I completely and cleanly remove it without affecting my new Matlab?
Deleting the older Matlab's .app file from your Applications folder should be all you need to do.
(There are apps for completely removing apps from your Mac. AppZapper is the most well-known; I use AppTrap, which is a free and IMHO better alternative. These apps remove system files etc. associated with the app you're deleting. In this case, however, since you're effectively updating and not deleting Matlab, you would probably want to keep these files.)
Finally found out that just deleting the app file is enough, but ONLY AFTER you deactivate your copy of MatLab before you remove it.