A user in the Blogoscoped forums reports that in his email, he's now seeing ads between messages -- a more in your face approach to ads in Gmail.
This promotion for a flight France-Iceland appeared just above an email confirming my booking of a France-UK train ticket (Eurostar). Contrary to the classic side ads, I did take a look at this discreet and smart ad.
I guess I won't skip those new ads any time soon, since they're directly inserted in my thread. I hope Gmail will never try to include banners here!
[image from Google Blogoscoped]
They look just like the ones that appear at the top of your inbox -- but instead, between messages and likely cannot be disabled.