Wired News reports that some companies are blocking blogs at the firewall:

Robert Mason (not his real name) would love to spend a few minutes during lunch catching up on blog posts from around the web, but his company doesn't allow it. The financial institution where Mason works as a vice president has security filters set up to block access to -- among other things -- any website that contains the phrase "blog" in the URL.

What's more, says Mason, such practices are becoming prevalent in corporate America, particularly in financial services. Mason sits on a roundtable privacy group of 20 of the country's largest banks. "My best understanding is that my company's anti-blog stance is the industry norm," he says.

If that's not a sign of mainstream arrival, I don't know what is :)