I admit, I have never heard this position taken before. I can understand that it could mean not forgiven in the sense of positional forgiveness (as opposed to judicial forgiveness), though for my own knowing, how do you reconcile this with Mark 3:29 calling it an "eternal sin?"
There is an important point I think we need to make in regards to the nature of this sin. The context shows what they were doing:
And the scribes which came down from Jerusalem said, He hath Beelzebub, and by the prince of the devils casteth he out devils. (Mark 3:22)
They were attributing the working of God to satan. When a person sees God at work (the finger of God) and they refuse to acknowledge it as God they are moving in the same error we read about in Romans 1:21;
Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
They would not acknowledge God. They saw the clear evidence and would not sign-off that it was in fact God. This is very dangerous because the heart becomes hardened towards God. Keep in mind that faith is response to grace; that is, faith is right response to revelation. (R.B) When God reveals Himself and the person refuses to acknowledge that the revelation is of God they cannot exercise faith.
What is worse is that these men were actually saying that Jesus had an unclean spirit, when the spirit He had was the Holy Spirit. I am afraid we risk losing the severity of this passage in modern times. When God says a person is in danger of hell fire we ought to take notice. They were saying that Jesus was moving in the demonic and that put their souls at risk. Now I'm not sure what Saul of Tarsus was doing when he caused people to blaspheme:
Who was before a blasphemer, and a persecutor, and injurious: but I obtained mercy, because I did it ignorantly in unbelief. (I Timothy 1:13)
If Saul was a typical Pharisee he would have likely suggested that there was a demonic source for the miracles of Christ and the Apostles (that none could deny were happening). Yet there is an out here... I did it ignorantly in unbelief. Saul was doing what his mentors were doing. He was probably repeating statements he had heard in ignorance and unbelief. So he obtained mercy. This is why we have to be very careful not to suggest that folk are moving in the demonic that are truly moving in the Holy Spirit. Not only does one risk their own damnation, but they also risk the damnation of those that follow their example. Fortunately for Saul he found out before it was too late. Many of his peers were not so fortunate. They fled to Yavneh and essentially continued the practice.