I can hear you asking: "What the hack is that?" Let me quote the Sculpin's authors:
There are a lot of places, where you need client's IP address in Drupal (or any other CMS/web app of course). The problem arises, when you use a reverse proxy server (like Varnish), since every request to web server will be done by the latter. We will have every single visitor of our website coming from a single IP (reverse proxy), as a result.
Slovenske novice are Slovenian national daily newspaper with largest circulation on national level. Last week we lunched redesign of it's web variant, which is a news portal with about 200.000 unique visitors and 4.5 million impressions per month. This numbers wil definetely become larger soon, as they have monthly growth between 10 and 15%.