I agree that Stephen's statement is most clearly understood as an affirmation of Jesus' Deity:
- He is quoting Psalm 31:5, addressed to Jehovah
- Eccl. 12:7 indicates the spirit returns to God
- The parallel declaration by Jesus (see Luke 23:46, written by the same author) directs this statement to God
In recent reading linked from another question I found interesting the argument that this statement by Stephen indicates that he expects to be immediately with Jesus. While this is possibly true, I do not believe it is demonstrated by this passage.
As noted in other answers, Stephen's parting prayer is very similar to Jesus' statement:
Father, into thy hands I commend my spirit (see Luke 23:46)
And given that Acts is part 2 of the Gospel of Luke, the author clearly noticed this. But did Jesus return immediately to His Father and/or to heaven?
John 20:17 gives us good reason to believe He had not:
Touch me not; for I am not yet ascended to my Father
Jesus' ascension to His Father would still be a few days away (or at least 36 hours) when He made the statement on the cross. As such, the statement at the very least implies eventual reunion but not need imply an immediate reunion.
Whether Stephen expected to immediately be with Jesus is not stated; he believed that he would be with Jesus at some point.
Stephen's extensive discourse on the history of Israel (see Acts 7), and the description of him as "full of faith and of the Holy Ghost" (see Acts 6:5) suggests he was well-informed in matters of theology, and would have known the Jewish/Christian teachings on the afterlife. There is no reason to believe he would have held views on the afterlife strikingly different from his faithful contemporaries.
I have written on the contemporary & near-contemporary Jewish & Christian beliefs on the afterlife here and here.
The verb translated "receive" is δέχομαι ("dechomai"). It is the same verb employed by Jesus in the Mission Discourse in Matthew 10. The parallels between Stephen & Matthew 10 are so striking I think it very likely Stephen has this teaching in mind (whether through oral or written record is a matter for another question).
Let's walk through several of the verses in Matthew 10 to observe the parallels:
17 But beware of men: for they will deliver you up to the councils,
and they will scourge you in their synagogues;
18 And ye shall be brought before governors and kings for my sake, for
a testimony against them...
That's what is happening in Acts 6-7--Stephen is brought before the council.
25 It is enough for the disiple that he be as his master
Stephen is walking in the footsteps of his master:
- Being accused of blasphemy against God & holy places (compare Matthew 26:61-65 & Acts 6:13)
- Pleading for the reception of his spirit (compare Luke 23:46 & Acts 7:59)
- Asking for forgiveness for his attackers (compare Luke 23:34 & Acts 7:60)
- Dying for the truth (compare Luke 23:46 & Acts 7:60)
28 And fear not them which kill the body, but are not able to kill the
soul: but rather fear him which is able to destroy both soul and body
Stephen is indeed fearless before his accusers; he is not dissuaded by the physical harm they can (and do) inflict upon him.
34 Think not that I am come to send peace on earth: I came not to send
peace, but a sword.
This is actively happening during Stephen's sermon.
36 And a man’s foes shall be they of his own household.
Stephen and his audience are of the house of Israel--he has just recounted the finer points of their common family history--and it's not the Romans, but members of his own house of Israel, who are attacking him.
38 And he that taketh not his cross, and followeth after me, is not
worthy of me.
39 He that findeth his life shall lose it: and he that loseth his life
for my sake shall find it.
Stephen is essentially taking up his cross here; he is quite literally losing his life for the cause of Christ.
40 He that receiveth you receiveth me, and he that receiveth me
receiveth him that sent me.
Same word, dechomai. Usage: "properly, to receive in a welcoming (receptive) way" (see here). Stephen has received Jesus, and now is asking Jesus to receive him.
32 Whosoever therefore shall confess me before men, him will I confess
also before my Father which is in heaven.
33 But whosoever shall deny me before men, him will I also deny before
my Father which is in heaven.
Stephen uses his second-to-last breath to confess Jesus before men (his final breath being a plea for forgiveness for his attackers, see above), trusting Jesus to confess him (Stephen) before the Father.
Whether or not Stephen expected a reunion now or later, he knows that his future is secure in Jesus' hands. Far more important the precisely when Stephen will meet the "Just One" (see Acts 7:52), Stephen knows that he is prepared to meet Him.