Just take a look at the Content Description Record in your ClientRegistry.blob or through the Steam client Console. There are two applications with interesting names :
Counter-Strike: Source Beta Content (261)
Counter-Strike: Source Beta Content2 (262)
If you take a look at the appinfo record for the second one, you get this :
AppID : 262, ChangeNumber : 21177, last change : Sat May 08 13:59:40 2010, State: 0
Section extended :
"extended"
{
"dedicatedserverfolder" "orangebox"
"cddbfingerprint" "1263160873"
}
And then you know CS Source is coming to the Orange Box version of the Source Engine