Dave G-J posted a very thoughtful comment on the post below on being filled with the Spirit and whether it is uniquely at conversion.
For a very thoughtful consideration of the question it is well worth a read of John Piper's sermon You will be baptized with the Holy Spirit here:
Here is an excerpt, but I recommend you read the whole thing. I agree with it.
I think that being baptized with the Holy Spirit (the way Luke means it) is not the same as being born again or being united to Christ by the work of the Holy Spirit. In other words I don't think that what Paul is talking about in 1 Corinthians 12:13 is the same as what is happening here in Acts.
Paul says, "For by one Spirit we were all baptized into one body—Jews or Greeks, slaves or free." The context shows that he is referring to a work of the sovereign Spirit who unites all believers to Christ. This is virtually the same as the work of conversion. When you are born again and put your faith in Christ, the Spirit of God unites you to Christ so that you are part of his body and a fellow-heir with him of eternal life.
I used to just assume that Paul and Luke were talking about the same thing when they used the word "baptism" and connected it to the Holy Spirit—in other words, that the baptism by the Spirit in 1 Corinthians 12:13 and the baptism with the Holy Spirit inActs 2 were the same. Many very able scholars and teachers still make that connection. The view I am about to give you is not the only orthodox one, nor is it one you have to agree with in order to be a part of this fellowship. But it is one that I am increasingly persuaded is correct and desperately needed in the church.
Receiving Extraordinary Power for Ministry
We are trying to answer the question: What is the heart or essence of being baptized with the Holy Spirit? I have said that I do NOT think the essence is new birth or conversion or being united to the body of Christ. What then is it? And why do I not think it is the same as what Paul speaks of in 1 Corinthians 12:13?
I think the essence of being baptized with the Holy Spirit is when a person, who is already a believer, receives extraordinary spiritual power for Christ-exalting ministry.