What do they signify? Solidarity with Palestinians? The influence of Western hipster fashion? A resurgence of secular pan-Arab identity, that several observers have noted? I think it's all three, and probably others that I can't think of right now.
In any case, here's a photo that I very much like, from a post on the blog of The Economist. (It's not credited to any photographer.) It seems particularly iconic. A young Egyptian male (the youth have played a critical role in organizing and activizing the uprising--although all segments of society have participated). He's carrying an Egyptian flag (signs of nationalism have prevailed over assertions of religious identity in this intifada). A cellphone is in his right hand (mobile phones & tweets & text messages have played a significant part in the movement, so critical in fact that the Egyptian government cut all cell service on February 27). He is walking in Tahrir (Liberation) Square, the focal point of the movement (although significant mobilizations have occurred all over the country, Tahrir, for all kinds of historical, cultural, and spatial reasons, remains very critical). The Nile River (Egypt's lifeblood) and the Egyptian Museum (the repository of the country's antiquities) are in the background.
And a black-and-white kufiya is draped around his neck, sign of global style, Palestine solidarity, and pan-Arabism.
Long live Egypt and its courageous and humane people.