This morning, I noticed a new ad unit within Gmail. The usual practice for Google, and almost everyone else, is to make the click open a new window and show the target URL in that. In this case, the ad unit opens a first level of content within Gmail, just like an email, and then there is a further click-through that takes the user to the final page.
Gmail now has four ad units in the mail content page and Google has been fairly innovative with their formats. One of the units — the top one — starts as a band on top and when you scroll down it collapses to a smaller unit on the right panel, above the usual set of four link tower unit.
As it is always possible, Google could just be bucket testing this unit.