This is something I've wondered and it's how most people promote aff. offers (whether it be CPS, CPA, CPL). personally I use a mix of organic and PPC, using PPC first to test out an particular offer in a niche prior to doing all the work for SEO (of course to ensure conversion). How ever I haven't done much (actually any) Social networking, viral marketing, or article marketing (with direct aff. links I use this for link building sometimes). Just like to get other peoples input on how they manage their traffic for their aff. business.
I think I've read around the forums that organic traffic converts better. However, I'm not sure if it is true.
If you can get organic it does convert better. The problem is that it takes too long to get organic. To survive that, you have to do a mixture of everything.