You asked for it, you got it! You can now opt out of being visible online. So lurk away!
There's a new setting. When logged in, go to My Home. Click on Settings in the left hand nav and you should see, under General, "Allow others to see when you're online".
Note though, we're not just showing you online for the no reason (well, right now I guess we technically are :). We're working on some features to allow members to interact in real time (hopefully chat - still in R&D), field questions from potential clients, etc. - if you're not visible online you probably won't be able to use these features when the come out.