I think it is. It's too bloody similar and Google likes to use OS projects.

I don't think so. Similar in design in some ways, but also different in many other ways. Maybe some inspiration, but different software.

They both have a stream of posts from you and people you follow. But so does twitter. It's true that Aspects and Circles are similar...

but they are both just the obvious solution to the problem of "I want to share some posts with certain people, but not others."

And I suppose you could say that Diaspora's tags are similar to G+'s sparks. But they are both just adaptations of Twitter hashtags.

Aside from those three things, which any software in this genre would have, I don't really see many similarities.

Google has started working on G+ over 3 yeas ago - which has been confirmed by several sources so far.

And I agree with Jacek - there are only so many ways you can do that - and both G+ and Diaspora are basically "Facebook done right".