WebMynd (also YC, incidentally) replaced Google.com adverts with content and Google never minded - WebMynd has lots more users and funding than Rapportive, and has been around for many many more years.
Also, when Google released their GMail gadgets marketplace, Rapportive was officially recommended on their blog, and I think it was one of the first on the scene.
Also, didn't that Paul guy who created GMail invest in them? I think he would have prodded the GMail team in his due diligence.
Given that GMail has millions of users, it's highly unlikely Rapportive will ever cause damage to whatever revenue GMail creates from adverts (its highly unlikely - sadly - that Rapportive will ever get, say, 5% of gmail users using Rapportive - the number is simply too big) - I imagine most of GMails revenue is from users paying for larger inboxes. Because they spend all their time in their mail client. Making them the dream Rapportive customer.
"Google shuts down service hundreds of thousands of GMail users love so they can make more money" doesn't sound like a very nice PR campaign. I suspect GMail is all about keeping users loving the Google, rather than monetizing.
I doubt Google will ever blog "hey guys, replace our adverts" because i think shareholders might raise eye brows, but turning a blind eye towards revenue in favour of delighted users sounds like a fair trade off.
Also if Google did make a move, it would cause so much press to flood towards Rapportive they'd be rather happy. (jinx)
I'd like to see people remarking how much users love Rapportive (search twitter) or how brilliant their customer service is, rather than what might cause Rapportive to fail.