UPDATE II: A leftwing take on today's events with many photos.
UPDATE: Islamists in al-‘Arish have attacked a police post; several killed in clashes. Situation isn't very clear.
Today was supposed to be the "Friday of Unity" in which all the supporters of the Egyptian Revolution would gather in Tahrir to show the unity of the Popular Will. It didn't work out that way. Islamists, including the Muslim Brotherhood and Al-Gama‘a al-Islamiyyak, but also more extreme Salafist groups as well, took control of the demonstrations and chants. By the end of the day a coalition of liberal and secular parties and the Revolutionary Youth Movement declared a boycott of the protest and demanded that the Muslim Brotherhood express support for a secular state. Al-Jazeera English has analysis here; while AhramOnline provided live updates through the day.
What it all means may take a while to become clear, but it looks as if the (always rather awkward) alliance between the Brotherhood and the liberal parties may be at the breaking point. Zeinobia's commentary suggests the Brotherhood let more radical Salafis have the stage (some displayed pictures of Usama bin Laden and Sheikh ‘Umar ‘Abd al-Rahman) in order to make the Brotherhood look more moderate, while SCAF may have encouraged the Salafis in order to justify its own role (the old Mubarak era "look at what the alternative would be" argument to the West). Maybe that's in fact going on here; certainly this may be remembered as the day when Islamists and secularists clearly parted company.
If I try to grasp for a positive spin on this — and I admit I may be grasping at straws — it would be to argue that this was the day the Islamists overplayed their hand, and may have inadvertently spurred the secular camp to forge a new unity, instead of bickering among themselves.