I have to argue the question, because I've been the one to say, "That's impossible!" and then turn around and write code that does just that. Really, it comes down to your ability to convey your needs and concerns. Like Sara said, give them a time table that makes it less appetizing and maybe they won't ask for it. However, they might find someone who at least says they can do it. At that point, your opinion probably won't matter because you'll be finding a new job. ;)
My experience is that if you explain why it isn't practical, possible, or just plain silly you are more likely to get a positive response. Now, working with designers you often have to use more common language which can be difficult for us programming types. You have to think about what you're going to say way before you say it, and that helps you to formulate why it really is wrong.
You win some, you lose some.
And, its funny you post this, I have a friend who does just that: Suggests crazy, outlandish things that are just plain inconceivable. Funny thing is, they always get done!