I love Dilbert
I thought the cartoon at http://www.dilbert.com/comics/dilbert/archive/dilbert-20070824.html was particularly funny.
I will add though that I've never had this problem at any time in my 28 year career. Clients have always understood priorities and that you can't do everything at once.
One client did hire a jack*ss in middle level management who thought he know a little about software development. After a few months I gently fired the client. I later found out that not only did they fire him they escorted him out of the building with security guards.
I also particularly enjoyed the following Orphaned Projects xkcd - A webcomic of romance, sarcasm, math, and language.