I've noticed a lot of news and adult sites have a "splash page" that you land on when you come to the site, regardless of the page you land on. On the news sites this typically has an ad that you sit through, while adult sites have an enter button.
I'm wondering how I can create that for my site and how to do it properly to prevent any search engine misgivings.
Theoretically there should be several different ways to do this. None of them are ideally perfect as there are some sites i just can't seem to ever get pass the splash page. Implementing them would be easiest if there is a plugin to do all the work for you. If you are going to write your own, here are some top level items you can look at:
1) Check for a cookie. They don't have a cookie, they haven't been to your site. Redirect them to your splash page
2) Check their referrer. If the request contains a referrer other then your site, redirect to the splash page.
I don't know if there are any stats for the number of visitors who don't go beyond a splash page. I don't use one myself but I have regularly seen visitors leave my site if a search doesn't land them on the exact content that they queried. They search for "red widgets", land on the "widget overview" page and perfer to leave than make a choice between red or blue widgets.
I would never recommend using a splash page unless there was a positive business need such as an age restriction.