I was thinking about this the other night. What was the conversation like between Obiwan and Owen Lars about taking in Anakin's child? Did Obiwan tell Owen what happen to Anakin? If he did, why would Owen put himself and his wife at risk if Vader found out his son was alive? At the end of ROTS we see Owen and Breu very happy to see Obiwan and accept the baby. What made Owen become so negative towards Obiwan? Why didn't Obiwan just raise the baby by himself or at least setup an agreement to train Luke as he grew up?
I'm not sure this has been clearly explained anywhere, but my take on it is this: Owen & Beru are more than happy to take on an adoptive son. Maybe they can't have kids of their own, but Anakin is family of a sort and so why not take on his boy to honor Shmi if nothing else? They obviously raise him/love him like a son despite retaining the aunt and uncle title. When Luke talks about joining the rebellion or seeking out Obi-Wan, Owen is just pulling the crusty old "stay and help onthe farm" bit to prevent Luke from going off to war. If you knew the boy you raised as a son was likely heading off to a life of fighting, you'd probably want to keep him sheltered as long as you could. I don't think Owen is so much negative at Obi-Wan as he is just scared of Luke running off and getting hurt if he knows the truth about his dad.
I brought up the reluctance of Obi-Wan to train Luke over in the Yoda thread, but totally agree with you there. I don't understand at all why Obi-Wan wouldn't be more involved in Luke's training at a younger age, especially after what happened with Anakin after starting his training so late.
The other piece I don't understand is why they didn't change his name from Luke Skywalker to something else if they were trying to hide him. Why not call him Luke Lars until he finds out he is truly a Skywalker?