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Below is part of an email I received from google themselves after I reported a site that was using both on the same page...
AdSense publishers may display third party advertisements on sites and pages showing Google ads as long as the formatting or colour scheme of these ads is sufficiently different from the layout of the Google ads.
In other words, if you choose to place non-Google ads on the same site or page as Google ads, it should always be clear to the user that the ads are served by different advertising networks and that the non-Google ads have no association with Google. If the formats are naturally similar, we would ask that you use different colour schemes for the competing ads.
Anyway, it would be a good idea to double-check whether Yahoo allows this. Google does allow this, but the last I heard Yahoo did not.
[edited by: MikeNoLastName at 1:55 am (utc) on Jan. 29, 2008]