Recently I've seen two amazing keynotes: "Down the Rabbit Hole: Lessons Learned combining Career and Community" by Cal Evans at PHP South Coast, and a couple of weeks before that I've seen "Hacking Foresight from Hindsight" by Josh Holmes at the DPC. I believe that what you take away from every presentation is very personal and depends on what you do and where you are in your life and career. For me, what I really learned from these presentations is funny enough the simple fact that it's ok to admit you don't know something, and that you will only benefit from being modest. (I'm saying funny, because this wasn't what the presentations were actually about).
In practice, this is harder than you might think. It's very tempting to keep up appearances and not to show any weakness. Maybe the person who is paying your salary is there as well, or a conference organizer who just paid for your flight, but most of the time we simply don't want to appear stupid. We like to show what we can, and it's hard to show weakness by admitting that there is something that we don't know.
However, by doing so you're missing out on opportunities to learn, and so you will probably always remain stupid ;-) . Software development is always moving forward so quickly and on so many different fields that it's impossible to know everything. The people at the table won't think you're stupid, because they don't know everything either. Yes, they will probably be more specialized on certain subjects, but if that's the case then that is actually an opportunity to learn - gaining their knowledge, simply by asking them to explain it. The people I've met are usually more than happy to share this information with you. In fact, most people will be so eager to explain the thing that they are passionate about that they won't shut up for the next 30 minutes or so.
You now know what they know as well, simply because you were prepared to set your ego aside and ask. It takes some guts to show weakness, especially if you aren't used to it - but if you're with the right people you can, and they will only respect you for being honest with them. If they don't? Then you're probably hanging with the wrong crowd.