I don't actually agree with the concept of predestination, but that's another topic.
Heb.6 is not IMO talking about someone who dabbled in going to church or someone who thought about getting saved (like the Roman, "almost persuaded"). The verse SAYS "Partakers of the heavenly gift..." that "fell away" and could not be "Renewed into repentance seeing as how they would crucify the Son of God afresh"...
How could you be "renewed into repentance" unless you had already been there?
If you are "partaking" of something you are swallowing it, you are accepting it. That's sounds to me a lot like salvation. Why would Paul (or the anonymous someone who wrote Hebrews *wink*) even find it necessary to tell people., "Yeah, if someone backslides he can't come back..." when that is 1. not always the case as many hear will atest and 2. quite obvious.
I think we have to take the verse at face value, not twist it around with theology and what we would like it to mean. The verse is obviously saying that those of you who once believed, but fell away cannot be saved because Jesus can't die twice. He can't forgive those who were once "enlightened" with the real saving grace and then deny it.