I have to wonder if this problem is somehow server related. I did had this problem, even over multiple restarts of Aperture. I fired up Aperture this morning and the sync worked.
A couple of things that may contribute:
1. This is a start of a new month. I wonder if that played into it.
2. I rename my Photo Stream Project so that they will display in order alphabetically. Can't say if I had renamed a project of an active month previously, but I know i did that for July. The funny thing is my renamed July Project still synced correctly. In the future, I think I'll wait until the month is over to rename the project.