Not impossible, though I'm guessing gmail is used widely enough that it would be a PR nightmare if they did this, much worse that the greader fiasco. They're probably forced to keep gmail alive, whether they want to or not.
I've been using gmail exclusively on my iphone for years with no ads. They've lost whole dollars providing this service to me.
But they want to make money off me, not lose me as a potential revenue.
So my bet is this is in preparation for showing 'sponsored' emails in your imap/pop3. "If you don't like it, just download all your emails and use another service" will be a talking point to suppress outrage, knowing that few people will actually go to the trouble of changing their email address. Fanboys will lap it up.