OK so it'd no longer be a secret now that I've let it out into the wild. Anyway, the hypothesis goes somewhat like this: the number of Gmail invites you get (to send out) is positively correlated to your usage of Gmail. "Usage of Gmail" is hard to define, but I reckon it'd include the number of emails you send/receive, and possibly the amount of storage you use (out of the 1GB).
Why do I say so? Um, if you've been following my inane writings on this weblog, you'd probably noticed gExodus, a Python hack job that I churned out yesterday that imports emails (in mbox format) into Gmail. It does this by forwarding your existing mbox - i.e. it parses the mbox file and sends each email via SMTP to your Gmail account. This registers as received mail in your Gmail account.
In my testing of gExodus, I probably forwarded around a thousand emails. I just logged into my Gmail account and saw the red "Invite a friend to join Gmail!" link...
And I had 6 invitations (I'm left with 4 now). People don't normally get 6 invites in 6 days' time. But 1000+ (received) emails in 1 day, 6 invites the next - all that's left is to connect the dots.