Let's talk validation...

This isn't a post about ways to validate, I think we're all sick of talking about the standard playbook / landing page / email list, etc. What I'd really like to discuss here is what are some of the metrics that you use to call your concept "validated". I'd love to hear your thoughts on B2B vs. B2C validation. For me, it's much easier to say a B2B concept is validated if you have a few letters of intent from buyers at $X per month. How would you go about metrics on validating a B2C concept?