Yes, my former work were very clear with our clients about this (usually Drupal as this was a couple of years ago): "we use open source because it means that if you decide you don't want to work with us any more, you're not locked in and can do whatever you want. It's also common enough that you can get your own in-house developer if you really want to add functionality to it."

Our strategy was that by informing the client, they a) knew we weren't backing them into a corner and b) were informed enough to have options in the future. (Most stayed with us anyway, but it was nice to say that they had a choice)