...continued from Wednesday.
When you put the question that way, Acts is pretty clear in its portrayal of being filled with the Spirit as something that happened at about the time when someone believed in Jesus as the Christ.  Acts 2:38 puts it this way to the crowds in Jerusalem on the day of Pentecost: "Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit." The verse seems fairly straightforward. You repent. You get baptized. You receive the Holy Spirit.
So Acts 2:38 seems to present receiving the Holy Spirit as part of the initial process of becoming a follower of Christ. Since Peter is explaining the filling of the Spirit that has just taken place (2:4), we have every reason to believe that being "filled" with the Spirit is the same as what he means by "receiving" the Holy Spirit. If we go through the rest of Acts, most of the Spirit-fillings happen to people who have just believed on Jesus.
In Acts 8:14-17, some people in Samaria have been baptized, but they have not yet received the Holy Spirit. Arguably, this situation is so unusual that Peter and John go up to Samaria to address the problem. They lay hands on those believers so that they can receive the Holy Spirit. In Acts 9:17-18, Paul is not yet baptized when he receives the Holy Spirit, then he goes on to get baptized.
In Acts 10:44-48, Cornelius and a group of Gentiles are hearing the good news about Jesus when they receive the Holy Spirit. Accordingly, they are then baptized. In Acts 19:5-6, some followers of the teaching of John the Baptist have not heard about Jesus. After they hear about Jesus, they are baptized in his name and then Paul lays his hands on them to receive the Holy Spirit.
In all of these instances, people are filled or receive the Holy Spirit around the time that they begin to follow Jesus and are baptized. Nothing in these stories indicates that receiving the Holy Spirit is an experience Christians normally have at some distance in time after believing. Indeed, in several instances people receive the Holy Spirit before they are even baptized.
At this point I should probably address those from the Wesleyan tradition, since holiness groups have traditionally associated Pentecost with an experience we call "entire sanctification." In the Wesleyan tradition, entire sanctification refers to an experience where God empowers a person to live a Christ-like life on a whole new level. For most of the history of the Wesleyan tradition, the idea of being "filled with the Holy Spirit" has served as a powerful image of God's empowerment to delight in making the right choices when temptation comes.
And why not? In Acts 4, when the apostles are facing opposition, a group of believers get together and pray for God's help. In response, God sends the Holy Spirit and they are filled yet again with power to speak the word of God boldly and to perform miracles in his name.
It is always appropriate to ask the Holy Spirit to fill us when we are facing some challenge or need! Have you ever felt like you were struggling to do what you know God wants you to do? Have you ever discovered areas of your life that you struggle to give to God?
The idea of entire sanctification, if we don't let the lingo get in the way, makes perfect sense. Doesn't Jesus need to be Lord of every area of your life? Isn't it a problem if we haven't given him everything? Wesleyans believe that the Spirit wants to empower us beyond a life of struggle with the same sins over and over again.
That's not to say new areas won't emerge or that we can't have relapses. It's just to say that the Spirit wants to give us the power to move beyond never-ending struggle. God wants to make us Christ-like. What better way to speak of God making it happen than filling us with the Spirit?
And most people will surely experience such a moment as a letting go, as an event of giving God everything. There is often a last hold out, some last thing you struggle to give God more than anything else. It makes sense that being filled with the Spirit in this way would often be an event, since we usually give out to God in a moment of decision.
If what we are talking about here seems foreign to many Christians, is that not a strong indictment of the church today? Could it be that the idea of giving every aspect of your life to God is so foreign to most Christians today that this whole discussion seems bizarre? Is it possible that Christianity for most Christians today is just something they do on the side, a nice little hobby to go along with the other parts of their lives? The idea that we might actually orient our entire life around God then seems like a novel suggestion, cultish.
But this is exactly what it means for someone to be a Lord...
 The classic examination of this issue is James D. G. Dunn's Baptism in the Holy Spirit: A Re-examination of the New Testament on the Gift of the Holy Spirit (Philadelphia: Westminster, 1970).