Safari 5 just might be immunizable too.
According to the developer of Safari AdBlock, Safari 5 (available for Windows XP and later) is able to block content: http://safariadblock.com/
Quote:
FAQ
Q: Does AdBlock for Safari actually prevent stuff from being downloaded, or just hide it after the fact?
A: The former. Chrome doesn't support preventing resources from being downloaded, but Safari does, and I added code to support that. This also means that SCRIPT tags will be blocked in Safari by the EasyList rules, whereas in Chrome we've had to constantly bandaid this with custom rules to block the elements the scripts put onto the page.
Whatever mechanism is exploited by this extension might be put to use by the Immunization feature in Spybot 2.0 (or at worst it might only be usable by those with AdBlock installed, by adding a custom filter subscription...which might also be a good idea to make available for the AdBlock extensions for Chrome and Firefox and lesser-known browsers).