Seems to me that the client model for processing a single vs. aggregated distribution might be quite a bit different. And also, the original upstream feed might have used entry/source already (this makes me nervous about the whole notion of PuSH co-opting <source> for its own purposes).
I was wondering if you might want to put an extension element here as a child of feed, before the entries start, in a pubsubhubbub namespace, saying "the following are aggregated by the hub". You can do this safely because Atom has MustIgnore on markup it doesn't recognize (hint hint). [Um, when I read this section, there's a little voice in the back of my head shouting "YAGNI!"] -T
