Two weeks ago, Facebook rolled out their new Places product; and, to drive user awareness and adoption, they prominently promoted it within their mobile application and web experience. This week, Twitter rolled out Twitter for iPad. It's an exceptional product and I encourage non-Twitter users to trial it as well... the user experience is that good!
To drive awareness of the new application, Twitter is delivering an interstitial to users browsing via an iPad. It's big, highly relevant and promotes the application over the web experience (just compare the size / placement of the 'download button' vs. the mobile.twitter.com link).
It's another great example of driving usage through relevant promotion (relevance defined by placements and user targeting)... and what I refer to as getting product "in the river".