There are some really good points being made in this thread.
I think that it is just as risky to switch from doing mainly Google PPC to relying on (mainly) Google
SEO. I believe that Yahoo and Live get a fair amount of traffic in the US but get very little in the UK. So Google dominates again.
It can be great to get that "free" search engine traffic but they can switch off the tap at any time. Look at the problems that Ray from Shopcodes is having at the moment with Google search engine traffic.
So you need to diversify your traffic sources to build a sustainable business. As well as
SEO and PPC on the 3 main search engines you have: email lists, RSS, links from relevant websites, social networks bookmarks, blog networks, press releases, viral marketing, link bait, creating really great content, offline Marketing (?), all the usual stuff.
The trouble with all of this stuff is that it can be very time consuming and out of your control.
PPC at least is very quick to set up and run and (apart from quality score B.S.)very controllable. You are using money as a tool (in place of time) to make more money. I think that is why big business wants to keep it for themselves and keep the little guy out. With the profits that they make from it, they can afford to pay good writers to churn out content and win at
SEO as well.