The New York Times reports that Sunnis attended
During the protest in Najaf, Sadr followers draped themselves in Iraqi flags and waved them to symbolize national unity, and a small number of conservative Sunni Arabs took part in the march.
“We have 30 people who came,” said Ayad Abdul Wahab, an agriculture professor in Basra and an official in the Iraqi Islamic Party, a leading fundamentalist Sunni Arab group. “We support Moktada in this demonstration, and we stress our rejection of foreign occupation.”
If Moktada seriously intends to call off the civil war (or at least remove the Mahdi army from it) that would be excellent. So he says he wants the US out. The US wants to leave. Where's the problem (aside from in the White House but that won't be forever).
I wonder if I can link to this photo with its remarkable caption
So I link one more time to Mark Kleiman, but take his ironic statement literally
"Moqtada al-Sadr just ordered his militia to stop attacking other Iraqis. It's a miracle! Civil war averted!"