At Soup.io (tumblelogging/lifestreaming) we initially had content creation buttons right on the front page. We figured that the best first action for new users

At Soup.io (tumblelogging/lifestreaming) we initially had content creation buttons right on the front page. We figured that the best first action for new users would be to actually post something, not filling out a sign-up form. But we had exactly the problem you're describing: People are so used to scanning for "register"/"sign up" that some even assumed we were in invite-only beta. We eventually changed that to a "Try it now -- no sign up required" button, which works better. The wording and UI is still tricky though: "Try it now", "trial account" etc. carry the connotation that it'll cost something later. We've been meaning to A/B test a straightforward sign-up form for a while and measure effects on both conversion & retention, but unfortunately haven't gotten around to it yet.