A few years ago, I did a SEO campaign for a website that was was largely dependent on Overture keywords, back when Google included the top three Overture listings at the top ofthe page. The site that I was working on had been around for a few years, but was so far down in the rankings on Google as to be non-existent. Within about four months of doing Overture, the website was popping in at the number one spot on Google on several of my registered keywords. There was no question that Google was influenced by Overture (even though the standard line was always, "You can't buy your way into Google rankings!").

I now have another site that I need to do an SEO campaign, and I'm wondering, Is Google still influenced by Overture? Or has Google completely severed all ties?

It is my understanding Google was never influenced by Overture/Yahoo results but the other way round. Yahoo used to use Googles search results, however, as far as I am aware, they have severed all ties and neither engine is influenced by the other.

Did you do organic SEO work to the site as well as PPC? It could just be it took a few months to be reflected in the SERPs and the fact that you were carrying out PPC work as well was a coincidence.

TBH I don't think seeing a change in results 4 months after starting PPC is the basis for a such a strong conclusion. Regardless, Nicky is right, Yahoo and Google operate independently of each other now.