The unpardonable sin is mentioned here:
And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. 32 Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come. (Matthew 12:31-32)
I think it is further explained here, by showing those who commit is make it 'impossible to bring them back to repentance: (This is a great description of how Judas responded to the tasting of the Spirit in his life as he cast out demons, etc. and followed the Lord in his ministry until gradually finding that he just did not believe)
It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace. Land that drinks in the rain often falling on it and that produces a crop useful to those for whom it is farmed receives the blessing of God. But land that produces thorns and thistles is worthless and is in danger of being cursed. In the end it will be burned. (Hebrews 6:4-8)
I think the idea is that a man can blaspheme God the Father, and the Son, but it is the immediate influence of the Spirit that makes the Father and the Son known to us, so if a person resists the Holy Spirit to an extreme they can't ever believe in Christ, making it unpardonable.
Trying to figure out who has possibly committed it is difficult because we can't ultimately say if a person died unrepentant or not. However, there certainly are some who seem like they have committed it. Anyone who seems to have been exposed to an outpouring of the Holy Spirit and has had a violent reaction against that work and continues rejecting every sign of God's work begins to appear to have committed it.
Some good candidates in my mind would be Judas, the two you mentioned in Acts, many of those figures in history that apostatized from the church and started their own heresies, and any other religious leader fully aware of the doctrine of grace and yet contain to deny it and assert some kind of works salvation, to name a few. However, no matter how much it may appear that a person has committed it, if they repent, they never did. Paul the Apostle would seem like someone who appeared to have committed it as he persecuted the church but later repented.
In the end, it seems rare then to commit this sin as it seems there must be a very intimate and powerful outpouring of the spirit upon someone followed by a very perverse resistance of that Spirit, such as in the case of saying Christ healed the sick by the power of the Devil.