"Posterous Spaces" sounds just like Google+'s "Circles".
I never used Posterous for blogging anyway, but surely this is bound to piss off the bloggers already using their service? Unless "Spaces" is just an extension to their blogging platform, and the title is misleading.
You can still use Posterous Spaces as a public blog. Each space can be public or private.
We're solving a similar problem as google circles. The basic difference is that Posterous Spaces are symmetric sharing so all members of a space can post.
I see your trying to leverage the existing posterous to do it, which is good from the perspective of getting it going but could easily alienate a portion of the user base that were sold on the dead simple blogging message.
Sure these changes may still allow them to do the same thing but the shift in terminology and organisation will be hinting to people the direct things are going which is away from their use case.
Granted this new niche might be a good direction, the HN crowd are hardly a good sample of opinions for something like this.
I never used Posterous for blogging anyway, but surely this is bound to piss off the bloggers already using their service? Unless "Spaces" is just an extension to their blogging platform, and the title is misleading.