I voted for the ICRA tag/other established method. I think ICRA is a bit complex, but, it is a standard that has been out there for 6 years. I don't think a simple 'adult' tag works well because some of the blogs have grey areas, etc. Some of the blogs written by teens would need the adult tag.
I also don't think the meta tag is the best method as hotlinked images, images referred to in emails, images passed around in instant messaging, etc aren't subject to it. I think rather than tagging the pages, there needs to be a way to have a robots.txt type file on the domain root that contains that info. That way, when an image is requested remotely, the browser or email client or whatever can quickly go to the root of the domain in question (or subdomain) and fetch an adult policy file. I don't support doing this in server headers as most people don't have the ability to change their headers.
In any case, any method is a good start as long as there is some solidarity among site owners.
http://www.w3.org/PICS/ is sanctioned by the www consortium and has been around since 98
http://www.icra.org/ is somewhat independent and has been around since 2000.
To make it work, there needs to be a concensus among one method that enough webmasters create a critical mass. If the adult industry starts self labelling pages, and linklists set a date where submissions after a certain date require whatever type of labelling, eventually you'll have enough that it will work.
perhaps even a resource list of where to go to register urls for some of the competing surfing tools along with a section to educate parents. There is a PICS compatible extension to firefox which is free. It only takes a few minutes to register a new domain with the few that still maintain domain name registries of 'adult' content.
http://www.safesurf.com/classify/ (they use PICS as their policy editor)
Whatever is decided, it needs to be easy to implement and implemented on a rather large scale. I've emailed a guy at google to see what they use to determine that content needs to be filtered by safesearch. If Google has a preferred method, obviously helping them make their job easier will go a long way.
I also don't want to see webmasters having to flex and support 14 different standards which is why I think that even though PICS is a little complex, it is probably the wiser choice since it has been around since 98, it can be very granular if you want it to be, or, you can have a simple 'adult' policy. Its supported by multiple caching & safe surfing software already and you're set.
There are 3 places that need to be checked with any system that is devised:
Browser Manufacturers (What does IE, Firefox, Mozilla/Netscape, AOL, Safari have in the way of plugins or support for content ratings)
Search Engines/safesurf (What do the search engines,proxy engines,surf protection software companies look at to determine the rating of a particular site)
Webpage Design Software (What formats are supported by popular webpage editors so that its not too complex)
If we knew how each of those 3 already handled content ratings and adapted to make sure that whatever method was chosen was already supported, it wouldn't require an infrastructure change on their parts and would just work.