I don't see a reason why it shouldn't be. As long as your website doesn't provide unaccepted content (porn etc.), you're free to do whatever you want with it.
They are allowed as far as they are not displayed in adsense format. (Text and display) We can display inline ads (Kontera, infolinks) or affiliates (amazon), or chitika or banner ads (Ozonemedia) But we can not use yahoo publisher ads which looks similar to adsense in the same page.