...Even though you're basically happy where you are?

I've been thinking for a while that it might be a good idea to look around, but how do you go about doing it? Do you hide the fact from your employer? What do you tell the recruiters? Can you play your current employer out against the new recruiter or vice versa, and how do you do that? Any pitfalls?

Edit: I've bolded the questions that people haven't really answered yet.