LinkedIn just introduced a new feature called "Open Candidates" - a way to signal that you're open to being recruited for a new job, without letting your boss know.  Historically, the only way to announce to your LinkedIn network that you were looking for a job was to literally make it the headline of your profile. Once you flag yourself as an open candidate, you'll show up in recruiters' LinkedIn searches, even if nothing else about your profile shows you're looking for new opportunities. 

Just go into your LinkedIn preferences and select the "Let recruiters know you're open" option. You can even choose what kind of opportunities you're looking for.  [Click link for further details.]