i've read that that happens around 6-8 weeks of pregnancy.
I've also been a CP checker (though have had some time off!! lol)...I didn't check my cervix first time around, so don't know exactly how it feels in a cervix that hasn't already been stretched to push a 9lber out!! haha
Do you check it every day? If so, you WILL feel a difference in the Os. For me, since I've already had one baby, the shape is more ovular than round, so more () than o...but if you're feeling the right place, you will feel a sort of dimple in the center of the end of the cervix. Are you sure you're not feeling along the side? Mine tilts, so the end is facing my back instead of more straight up and down, so if yours would happen to be similar, it could be a little more confusing to feel the Os without doing a bit of exploratory work!
I was able to find such a good diagram for someone on FF before, but can't seem to come across it now to show you the difference, but assuming you've never had a child before, this is what you'll be feeling for.
http://www.drstandley.com/images/std/Cervix.gif