I know that I believe in once saved always saved theology. I have had doubts about it in the past though because of certain verses. But I have since reconciled those for the most part. If once saved always saved wasn't true, I would have been permanently damned within months of first receiving salvation. I have had a tendency to backslide throughout my Christian life unfortunately.
But praise be to God that he brings me back everytime!
Seebs, this is the explanation I heard while going to a Fundamentalist Baptist Church for Hebrews 6:4-6. Here is the verses in the KJV:
Hebrews 6:4-64 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, 5 And have tasted the good word of God, and the powers of the world to come, 6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
The explanation that I remember (I may not have it fully correct as it's been a while since I was told this) was that it is only humanly impossible to restore one again to repentence and that it must be motivated by God before repentance has occurred.
I have also heard another explanation that basically says that these verses are referring to those who came very close to salvation but turned away at the last moment. I am a little more skeptical of this explanation because the verse seems to clearly indicate that they were truly saved.
Also, a quick Google search reveals a number of pages that discuss these particular verses.
God Bless!