Honesty is a great selling tool.
Clients appreciate honest, straight answers and can usually tell the difference between a designer who knows what she’s talking about and one that’s trying to sound like she does. If you’re in a meeting and out of your depth, say it. Be specific about which of the required processes or technologies you are not familiar with and offer a solution – either to outsource that part of the project or to use a different process or technology which would achieve the same thing. Clients love that. It tells them that everything else required for their project is within your scope of experience and for that assurance they might well be willing to scale down on the bit that is not.
I’m not sure why people don’t do the honesty thing as the default. It gets better results and, more importantly, it’s a better way to treat others. A better way to live.