Any form of easily-understood signal will help; the only requirement is that it not be rude to those around you. (Alternately, it can be so over-the-top rude that everyone thinks it's funny. But they'll also take the point.) The funniest one I've heard is two students in the sciences who were writing theses - they got a hold of a pair of white labcoats, and put them on when they don't want to be disturbed writing their theses. "I'm doing science".
I know one person who tried a Do Not Disturb sign. Like yours, it was a bit over the top, and that seemed to help. Again, worked with some of his coworkers, and not with others.
* As everyone else has mentioned: headphones.
* Work at home when you're doing a particularly complex piece of work.
* Stagger working hours so that there are times when not all employees overlap - start time is anything from (approximately) 8am to 10am.
* Set instant messenger statuses to 'away' or 'in a meeting' for critical work. (This one only works for people not within line-of-sight.
* Have team leads (approx 1 for every 6 people) who can answer more general questions and so act as a buffer between programmers and people who might need to interrupt them.
* Provide people with the ways to find answers rather than the answers themselves (This has the dual purpose of making it less convenient to ask you than to look stuff up, and teaching people to be more independent.)
* Have times when you are more clearly interruptible - this groups the questioning / passing the time of day, so you can get a longer uninterrupted stretch later.
edit: formatting
Green up meant you were open to interruption for anything.
Yellow meant you were open to interruptions for something work related.
Red meant don't bother me unless the building is on fire.
I was really bad at remembering to change flags, and was always on the lookout for some USB-driven traffic light thing that didn't cost a fortune.
1) Headphones + a sign saying "Do not disturb when headphones are in place (I used an ANSI sign generator which I can no longer find) 2) Come in an hour before everyone else
I haven't tried it myself, but have seen other people work from conference rooms, cafeterias, etc with a laptop and phone-forwarding.