SEO considerations around an "Ad Wall"

[IrvCo_Interactive]

I'm not sure what the correct terminology would be for this but I'm calling it an ad wall. Essentially an ad overlay when someone enters a website. I see this most commonly on certain news websites. For example when you click on a link to an article on ign or forbes.com you get an ad that you have to close or skip to read the article. What are the SEO considerations if implementing something like this?

I'm wondering if there are any similar to a pay wall in the sense that you want to let crawlers in to see your content and rank it but users get an ad or redirected to an ad and then back to the article page. This link currently does it for me for example

http://www.forbes.com/sites/tjmccue/2012/05/22/spacex-launches-with-15-dreams-onboard/

I set my user agent to google bot and go right through to the article but if it is set to the browser default I get to an ad page I have to skip first. Is this the infamous "white hat cloaking"? Are the other ways to implement the same idea (a modal window that opens via javascript for example) that are more or less risky? I'm mainly interested in doing this based on referrer: people who type a URL directly don't see it but clicking on a link they do see it, for example.

[CleverPhD]

Rand talks about this here

http://moz.com/blog/white-hat-cloaking-it-exists-its-permitted-its-useful

Key word is "may" be penalized.  If you follow Forbes example, you should be ok.  All due respect though, you are no Forbes and so while users may put up with it there, they may not like it on your site.  Watch your analytics as you may see users leaving your site more quickly.