In pull request #3418 to Condenser, the software that powers Steemit, Jonathan Porta proposed changes that would allow the owner of a Condenser instance (in the case of Steemit, that would be Steemit Inc.) to insert promoted posts into the trending tab. These posts would have a gray "Promoted" tag beside them. Here's what they'll look like:
These posts behave pretty much the same as the existing "Featured" posts
Presumably, you'd be able to get your post here by paying Steemit Inc. I wonder how much they'll charge for a spot.