I'm not currently in a relationship, but I can answer for times that I have been....
When I get into a relationship, I enjoy the process of going through all of my online profiles (myspace, connexion, facebook, here...) and changing my status from "single" to "dating" or "in a relationship" or what-have-you. Usually after he and I have had some kind of conversation about what our "status" is.
I don't really tell him about it, but I definitely do it as a communication, both to him and to other people. I want to make sure that if he DOES decide to "do a little research" into my online life, that it will make him smile to see that I have already marked myself as "taken" without him having to ASK me to do it. It's a gesture, one that I hope will make him feel good and help him to know that he can trust me.
I also feel comfortable with guys that I date having online profiles, especially on sites like this. There are some exceptions: Manhunt, for example, and gay.com. I fully admit to having a bias in that regard: if I'm dating you seriously, I will ask you to suspend/delete your profiles on those sites. But on sites like this one, or myspace or facebook or whatever, I have no problem.
Finally, I should note, that I've met a LOT of the guys that I've dated through social networking sites... so it wouldn't be able to be a secret even if I wanted it to be. LOL