You can use other contextual systems, just not ones that look like adsense. So, no, you can't use YPN on the same page as adsense.
its not google thats the prude...its Yahoo.....google allows it but the ads have to be different. Yahoo doesn't allow any other contextual based ads on the same page.
I think the reason is to increase their (adsense's) monopoly over the various PPC programmes, by trying to stop you using their competitors. DD
Actually, yes you can according to Adsense. You just need to slightly change the appearance of one or the other's link colors.
You would then be in violation at YPN, so still not on the same page. It is also questionable if adsense would allow it. They are rather vague in the TOS. Really depends on what they define as look and feel I guess.
According to the person I spoke with from Adsense, simply changing the font color is enough to differentiate.
What is better? Ticking off Google or Yahoo? Anyone enjoy more revenue over the other? I've not been accepted into the YPN so I have no answer to this.