How do you think this is going over? I think that if you invite people in,and they aren't under NDA, posting signs asking them not to blog probably isn't going to do much.
What's the subtext to the signs anyway? "Please don't blast us on your blog"? "Life is random, but please don't talk about us"? "I'm a Mac and I'm very private"?
If I were Steve Jobs, first I'd look around my house and say "no more washing the car and mowing the lawn for me! Whee!" and second, I'd start to think about what some of these things are saying to the market. Mystique is cool, but you can't manufacture it. Bloggers blog. If you don't want them blogging, don't invite them to an event and if you don't know which attendees are bloggers, then don't have the event and if you do and are still nervous, then don't release any proprietary info and if you are relaseing proprietary info, then have them sign a non-disclosure agreement. But please don't post signs. Gah.
"Welcome, leopards! Please change your spots before entering"
"Hi Customer Evangelists! We don't trust you"