Obi Wan goes into exile because he will watch over Luke. I don't understand why he didn't train Luke when he was young. If Luke was trained young, he could have easily destroyed the emperor and Vader. I have heard that the emperor would feel a new force user, but then why couldn't the emperor sense Obi Wan was where he was? Obi Wan definitely knew of the Rebel Alliance, because there were alot of people of Tattoine that were a part of it.
I figure that one of the main reasons Obi-Wan didn't train Luke from the get-go was that Owen and Beru didn't want the kid to grow up as a Jedi and get killed on some ridiculous adventure.
"There is something going on in time and space, and beyond time and space, which, whether we like it or not, spells duty." -Winston Churchill
"For of all sad words of tongue or pen, The saddest are these: 'It might have been!'" -John Whittier
"Apathy is death." -Kreia