Youtube in-video ad
 

Just got these first time few minutes ago. It's the same for everyone ?

Not for me. At least, I don't think so. I don't really know what you're talking about.

No more ads yesterday. I'm afraid they were testing them and soon everybody will see what I mean. The ad takes about 1/5th of the screen at the bottom and can can be clicked away, however a small icon remains to bring it back.

The ads have been running for a while, at least since February. They tend (exclusively? I've not seen one anywhere else) to be placed over premium partner content rather than user content. The sales sheet offers a flash overlay ad with or without a 300x250 side ad. You can choose one of 13 categories. The product is called Youtube Invideo Ads; Youtube Video Ads is a separate product.