Gmail's viral marketing

Mike Linksvayer was the first I saw to point out the cleverness of Google's "Invitation Marketing":

How many otherwise respectable folk have you seen dedicating email broadcasts and blog entries to announcing that they have a few Gmail invites to give away, especially in the last couple weeks? I lost count long ago. Ad avoidance overcome, indeed.

I don't think I've been on a mailing list where Gmail invites weren't offered. Is this what was meant by sell-side advertising?