Yahoo! has opened up its search technology to all comers via its BOSS service. I’ve looked over the BOSS licensing terms and the AdSense terms and conditions and it appears to me that it would be legal for now to display AdSense ads on pages that contain search results obtained via BOSS. At some point Yahoo! will supply its own ads (which more than likely will come from Google, ironically enough) and you’ll get a share of the revenue, but I think until then AdSense is a viable option for using BOSS.
If you think otherwise, please let me know why, I may have missed something somewhere in those documents.