I write a lot about product marketing and the concept of "in the river" promotion / marketing. As companies more aggressively test and roll out new products / variants to users - in the river marketing becomes more important. And for those reasons, it has also become more prevalent. Here is a great example from Google, who is pushing Google Voice within GMail. To push Google Voice, Google is providing free phone calls for all of 2011. That's an aggressive, compelling promotion.
So how does Google get the message across? They place is prominently on the screen where its users are absolutely engaged and can't miss it: when you open GMail: