Is Sora and Roxas the same people in kingdom hearts?

*NO SPOILERS*

Ventus (Ven) is a protagonist in Kingdom Hearts Birth By Sleep, which takes place about 10 years before Roxas came into being, therefore, Roxas and Ven could not be the same person. However, Ventus and Roxas look almost exactly the same with the exception of clothing, which indicates some connection between the two. All of this is explained in Kingdom Hearts Birth By Sleep which is out in Japan and is scheduled to be released on September 7th, 2010 in North America and September 10th, 2010 in Europe. I don't want to ruin the game before it comes out, so lets not spoil the ending.