If I remember correctly, it is against the ToS to place the ads on any sites that are not owned by the publisher... Sorry bud, but I doubt you could get away with that... for long at least.
well i don't see how anyone would ever know if this happened. i've seen several people on several boards both ask and offer to do this. but one thing to remember - the person with the YPN account will be getting a 1099 form at the end of the year meaning they will have to pay taxes on the $ they collect from someone elses site. It wouldn't make sense to do it for any less than say a 40% discount unless you 1099 the person whose site you are letting use the ads.
One way to go may be to exchange channel with others who don't have google adsense. In that way to some point that 1099 issue won't arise..