Doesn’t it do something wacky like parse the content of your emails for keywords and toss related ads at you?
Yep, in the exact same way that Google puts unobtrusive and generally relevant ads on the right hand side of your search results.
Compare that to Yahoo, which always presented me with a big-ass, flashing ad for crap I wasn’t interested in, using up a quarter of my Inbox page.
Oh, how I love Gmail.
Yes, but they’re GOOD ads. Some examples:
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A friend was coming from out of town, so all of us were sending e-mails back and forth (the threaded messages function making this incredibly simple) about the plans for the weekend. I sent a message asking basically, “where are we eating Saturday before we go see (some movie)?” and the reply was “I don’t know Baltimore very well; I could show you a hundred places in New York though. Let’s pick the theater and go with something close to there.” Alongside this message, I got two ads for Baltimore dining guides, and an ad for movie times. Despite the fact that both sets of keywords were there, I got no ads for New York dining guides! Pretty good parsing if you ask me.
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I sent condolences to a friend whose father passed away, along with my regrets that I couldn’t be at the funeral. One of the ads next to her reply said “Can’t be at the funeral? Send flowers!” which I thought was remarkably helpful. She has pollen allergies, though, so I didn’t.
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When a few friends of mine discuss Diablo II, I get ads for online item auctions.
It’s not intrusive, and a lot of times I get a laugh out of how accurate the ads are. If I wanted to, I could set my browser to block them – but I don’t, because they’re so well-targeted that I know I’m going to use them again. And probably soon.
Shoulda figured my weak rant would be become the home of some invites. 
I just figured out what to compare Gmail’s invites system to. It’s exactly like Cartman’s “You Can’t Come” technique that gets his amusement park thriving…