Exploiting the hosts file to swap out ads seems to be growing in popularity. A few weeks ago there was a thread about some website instructing visitors to repoint the adsense server to their own server in the hosts file.
Thanks for posting this! I would expect to get emails from concerned visitors to my site(s) if inappropriate advertising appeared. Then I'd hopefully be able to instruct them on removal of the Trojan from their computer. Fortunately, my content is all very family-friendly and my users would be quick to alert me if something was funny to them. If I hadn't read this post, I wouldn't know it was a visiting computer's issue and not something going wacky with my ads!