As a "buyer" I think it's an excellent idea.

There's a prominent developer in a community I follow who recently found his company to fold, and posted that he was looking on his blog. At the time, I was pitching for a project; I set aside a place for him and had I won the project, would have contacted him asking if he wanted the job.

In another case, someone's failure became a blog post, which I shared with management of my then employer, who authorized me to get in touch and offer the person a no-questions-asked job (she didn't take it).

If you're freelance/consulting. You could also say; available for booking/project work from September. (Wordsmith away!)

I think it's a pretty good idea, thanks for the suggestion. I just added it to the 'about' section on my blog: http://madebynathan.com/

Would it be a good idea to put "I'm looking for a gig now" if you are a permanent employee? What if your boss sees it?

I wouldn't advise this, seems like a terrible idea that could put you on side-projects/unimportant stuff in your current work or that could even get you fired, depending on your country.

I don't think any boss that notices the "I don't like my work/I am looking to leave" would be happy

Yes my thoughts too. Unless you are a contractor who is not getting renewed or the boss already knows you are leaving for some unchangeable reason (change of location for example).