I've been getting a lot of SNAM (Social Network Spam) so I'm happy to hear that LinkedIn has a new flag that you can set that prevents you from receiving invitation from people who are not in your address book. It's a bit snobbly, but it prevents you from having to turn down invitation requests from people you don't know. On LinkedIn, I generally don't accept invitations from people I don't know because the purpose of the network is to refer people to each other and you can't really write a reference for someone you don't know. Although this probably lowers the "virality", I think this feature will enhance my LinkedIn experience and hats off to Reid for implementing it.
For people who are on LinkedIn, here's the URL to set the flag. If you haven't uploaded your address book, this will effectively make it impossible for people to invite you though.[Joi Ito's Web]