I came up with the following fairly generic "sales pitch," with Ami's feedback incorporated. Please offer suggestions for improvement if you have any. And of course, feel free to use it, either as-is or in modified form, if you like.
Hi everyone... I wanted to bring people's attention to a new physics Q&A site that just opened up to the public.
The site is built on the same system as Stack Overflow, the most popular computer programming Q&A site on the internet. It's a system specifically designed for the question-and-answer format, rather than extended discussions. Anyone can answer questions, and the best answers are voted to the top; registered members who consistently give good answers are rewarded with "reputation" and increased privileges on the site.
Everybody is welcome to contribute; in fact, you can ask and answer questions with no registration required! But we're most interested in attracting experts: researchers, graduate students and advanced undergraduates in physics or related fields. We want this site to be a place where anyone with a question about physics can come to find quick, reliable answers.