There is a really informative post on Wolf-Howl today on the results of viral marketing. More importantly, Michael discusses how to retain that traffic long-term.
I have noticed the effect of surges in RSS feed subscriptions in the past, followed by a slow decline in readership after a few days or weeks. Some readers stick around long-term thankfully. Brian at Copyblogger has also been talking about RSS.
Defensible traffic is one of the reasons I placed a priority on subscriptions and community involvemnet on this blog, rather than monetization.
Here you can read more about my blogging strategy and my efforts to build this blog into a community.