The W. Hall Harris commentary on the issues says this:
The use of the verb ejnefuvshsen to describe the action of Jesus here recalls Gen 2:7 in the LXX, where “the LORD God formed man out of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living being.” This time, however, it is Jesus who is breathing the breath/Spirit of eternal life, life from above, into his disciples.
The idea presented in this commentary is not that it's the Holy Spirit, but rather the breath or spirit of eternal life. Just like God breathed life into man in Genesis 2:7, so Jesus breathed eternal life into man there in John 20:22.
The Harris commentary goes further, saying that it also evokes the imagery of Ezekiel 37:1-14 and the valley of dry bones. In particular, it seems to evoke this passage:
9 Then he said to me, “Prophesy to the breath; prophesy, son of man, and say to it, ‘This is what the Sovereign LORD says: Come, breath, from the four winds and breathe into these slain, that they may live.’” 10 So I prophesied as he commanded me, and breath entered them; they came to life and stood up on their feet—a vast army
It's very clear here that Jesus was giving the Holy Spirit:
John 20:22 (NIV)
And with that he breathed on them and said, “Receive the Holy Spirit.
It seems most likely that this breathing was Jesus giving them eternal life in the form of the Holy Spirit. This can be seen in the Genesis parallel as well as the allusion to Ezekiel.
This is further supported by the reference that made earlier in John:
John 7:38-39 (NIV)Emphasis added
38 "Whoever believes in me, as Scripture has said, rivers of living water will flow from within them." 39 By this he meant the Spirit, whom those who believed in him were later to receive. Up to that time the Spirit had not been given, since Jesus had not yet been glorified.
However, at this point in John 20, Jesus had been crucified (ie glorified (John 12:23)).
So, the Holy spirit was clearly being given here.
Reconciliation with Pentecost
We can see in Acts 2 the Holy Spirit giving power to speak in tongues at the day of Pentecost. Many presume that this when the Holy Spirit was given to humans.
The Harris commentary argues that the eternal life of the Holy Spirit was given in John 20 and the power of the Holy Spirit was given in Acts 2.
I personally think that John 20 is clear that the Holy Spirit was given at that point. While the manifestation of power was not given until Acts 2, that does not exclude the idea that the Holy Spirit had already been given.
John 20:22 was the point, post-resurrection (ie post-glorification), when the disicples received the Holy Spirit. This is the point at which they received eternal life through the Holy Spirit. And Acts 2 was the first manifestation of the power of the Holy Spirit.