I'm not sure if this is very new, but Google Reader shows Google's feed ads as text, not as images, like in all other feed readers.

http://img206.imageshack.us/img206/473/readertextads.png

I think it's not very new but not old. I've seen text ads several days ago.

To clarify, all the ads look like text, but they're usually served as images to bypass the restrictions from feed readers, which remove scripts, iframes and other potentially dangerous content. Google Reader made an exception and probably allows any content from googleadservices.com.

I've only seen ads like this appearing this week. (I noticed because they're blocked by the proxy at work which shows our corporate logo and an warning message in the iframe instead.)