updated 11:59 pm EDT, Mon July 15, 2013
Until XProtect updated, only cure is to reset browser
The ransomware page can be landed on or pushed to users who are using alternative search sites to look for certain kinds of keywords having to do with pirated software or pornography. The page appears to be from the US Federal Bureau of Investigation and claims that the user has been viewing or distributing illegal software or pornography, and that in order to "unlock" the computer they are obligated to pay a release fee of $300, using a fake URL that starts with "fbi.gov" to fool unsuspecting users.
Users can escape the scam by choosing to reset their browser. In Safari the command is located in the application menu and choosing all aspects of the reset. The action does not remove bookmarks but does clear out saved name and passwords as well as resetting any Top Sites that have been saved.
Apple has built-in malware protection software in Snow Leopard and later systems that was recently updated, but it's not yet known if it will successfully block this particular malware yet. Assuming it does not yet block the scam, the company is likely to update XProtect to avoid the problem in the near future. The hack does not yet appear to work on mobile browsers.