I sincerely doubt most of the posters on this site are going through the trouble of setting up proxies and routing their posting through services that guarantee internet privacy. Even then, a subpoena works wonders on garnering even supposedly secure online posting information. If you are not doing anything to guarantee your privacy all it would take is for your provider to be given a subpoena and voila! you are no longer anonymous.
But even though I am positive at least 80% of the posters here are simply logging onto their home or school accounts and using that to access the internet and this site, you don't read a thousand threads a day about being busted because of what they posted online. Most of the "busted" threads come about because the person told someone, and that person either ratted them out or told someone else who did.
If the cops had the resources to sift through all of the crap posted on this and other marijuana boards and then use that to build a case, well we are so far down the police state path at that point you might as well line up to get your social security number tattooed on your forehead and the government's tracking chip installed in your anus.
Now if you were the next Bruce Perlowin and had commercial grows with pictures of your crew harvesting and working thousands of plants .... well, you might be an idiot to post that online. But your closet grow? I doubt the DEA has the manpower or desire to track you down and bust you.
But hey ... you might still want to jack that unsecured wireless account your neighbor down the street broadcasts to everyone within range, or post through a proxy address that you connect to via a Starbucks location. Not saying I do that ... then again, not saying I don't either.