So Seth Godin coined the term "landing pages"? Cool. In this blog he talks about increasing the effectiveness of landing pages. I agree how critical it is. I find marketeers often focus on the initial ad and hope for the best with the landing page. It's amazing how even the smallest tweak can make several % difference. For example, sending the visitor to a page that is actually about what the ad was about (send them to the product rather than the store homepage), or making it obvious what you want the visitor to do (click "buy"!). People often look at me oddly when I suggest testing multiple landing pages - "This is our page and it was approved by the committee and the CEO, we are not changing it. Besides, 'click here' is a perfectly fine call to action. The flash intro only takes 15mins to play out.".
Here is another idea; if your site is there to make you money make your 404 page useful. Even if only one in a thousand visitors see it, they will thank you for it.
Mr Godin is the author of several of my top 20 marketing books. (Even though it is an ever changing list where flux is the only constant I am sure "Permission Marketing" would always be in there).