I've been a big fan of LinkedIn for quite some time, and since I abandoned Outlook (and LinkedIn's amazing Outlook plugin), I've been using the Firefox and Gmail extensions pretty regularly.  When I logged in today to invite a few of my contacts, I noticed I could import all of my contacts from my Gmail (and Yahoo, Hotmail and AOL mail) address book, and invite them at once.

From your LinkedIn home page, click on the "+Expand Your Network" button on the upper right and follow the instructions.  Very easy and very cool!