This is an interesting question that popped up in one of the WordPress groups that I am a member of.
Some of the forum participants suggested that the client doesn't need to know what software was used.
I personally think that it should be disclosed because the fewer surprises the better.
I agree that in most cases you don't need to disclose too many details about the solution you're going to use.
You're the expert and the simpler you can keep it the better.
I'd say the developer/designer has to make sure that they mention that the solution is a popular CMS that will save a lot of developing time compared to creating something from scratch. Also since it's a CMS the client will be able to make edits themselves later on.
Another very important point is that one should mention is that the software will require regularĀ maintenance e.g. to keep it updated (or otherwise it may/will get hacked eventually) and to make sure backups are running etc.
