I think a lot depends on what the service is, and how much information is asked for at registration. My client started out offering a free service. We requested first name, email, date of birth. Then the email was confirmed via a clickable url in a confirmation message before they became officially registered. I would estimate one in about 30 home page visits resulted in a sign up.
I think the popularity of BugMeNot could possibly become an issue, there is an extension for firefox that uses BugMeNot too. I use it all the time to access things like WashingtonPost and other sites that require senseless registration.