Some people have made some very good posts, but if it's ok, I'd like to add in my own.
This topic has been on my mind for quite some time, so I researched it and wrote an article on it. You can read the full thing
here, but I'll try to summarize it.
Basically, when Jesus talked about the "unforgivable sin", he wasn't talking about a single act. If he was, he'd be contradicting the rest of the Bible. If you examine the context and his audience (the pharisees), you'll probably have a more proper viewpoint.
Because no one has been born a Christian, we've all blasphemed God in our lives. However, God is still able to forgive us. Jesus was talking to a group of people who knew who he was, who had the proof of who he was, and still continually rejected him. Here's 2 examples:
Nicodemus was a pharisee. He fit into the category that Jesus was talking about. However, he did not continually reject Jesus. He ended up accepting Christ as the Savior.
Simon Peter. He rejected Jesus 3 times and basically told him that he (Jesus) was lying at one point (and then Jesus called him Satan). However, even he was not beyond salvation. He came back to Christ and later gave his life for him.
(It's already been said before, but...) Trust me, if you think you've committed the "unforgivable sin" and are worried, it's a good sign that your heart is repentant and you will still receive eternal life.