It will be interesting to see if this works. del.icio.us is a good demonstration of sharing and storing links with attached context but very few users use this aspect of it. I sense that many users do not have enough time or inclination for such advanced features.
Lightweight linksharing. Friendfeed, twitter, tumblogging, and now google enters the space.<p>I think this is probably too tied to gReader to make a real go of it... but this particular problem is ripe enough that I'm working on an idea in the space, too.
It's a nice feature but I doubt it'll get used much. For sharing to happen you need to first create a space where people enjoy hanging out with their friends, and that's not really what Google Reader is.