Well, it's Holy Week and Friday - and this verse has come up again as a topic of discussion.
I have to say that it was a very meaningful verse in my life, since it ended my relationship with my former fiancee about five years ago.
That anyone could believe that the text was intended to mean that Christ was promising presence in paradise with him on that day, was just too much to abide. That poseurs could come into my house under the guise of bringing "glad tidings" and argue anything else struck me the same way.
As a result much of what I have had to say on this forum has rested on that moment and the issue raised. Below is an example in which we had the good fortune to have good contributions pro and con. In this particular instance, I was examining Christ's statement in the context of other times he prefaced his remarks with "Amen, amen I say unto you..."
http://www.jehovahs-witness.net/watchtower/bible/235015/1/e2809cTruly-I-say-to-you-e280a6e2809d-e2809cAmen-I-say-to-you-e2809de280a6-Inferences-from-a-Gospel-manner-of-speech
If that doesn't work, try search Luke, Luke 23, Kepler and Luke...
Since we sit near 2000 years after the events of Christ's life and the various accounts of it, we have reason to wonder about all manner of things. Whether he lived as described or which account is closest to the heart of matters. And since there are four Gospel accounts, we have to wonder why that number. Clearly as testimony they have different perspectives. Matthew seems to finish the story started by Mark - and which we wonder whether Mark had ever finished. And Luke seems like the synoptic straggler, not as far afield in his account as John, but distinctly different, nonetheles - especially since he includes a sequel known as Acts (Now how come we don't have four versions of that?).
But the point I am getting to is that it is Luke that makes the claim that Jesus conversed with the two thiefs at all. The other two Gospel writers simply said that they mocked him (Mark 15:32 "Event those who were crucified with him, taunted him" - ditto, Matthew 27:44 with elaboration). There was no mention of an apology from either one of the thiefs.
The point is that Luke clearly meant to make this additon. If Mark's account says Christ's last words were "My God, my God, why hast thou forsaken me?" (Mark 15:33), Matthew adds that Jesus crying out in a loud voice gave up his spirit (27:50). If Mark did not recall any further utterances or Matthew neglected to say exactly what it was, then Luke supplies: "Father, into your hands I commit my spirit." Luke 23: 46.
Since we are taking testimony from ancient writers and we all have different points of view, nothing definitive can be said by any of us except on the basis of faith or lack of it. But it is clear to me that Luke INCLUDED in his testimony the specific details that he did with clear intent. The intent being that these details were not included in the other two accounts. Whether he just happened to notice that from memory or it made more sense to say these things on account of what transpired in the sequel - I leave that open to conjecture. Either way though, it was an important point to the third Gospel writer, perhaps important enough to compel him to write it the account in the first place.
So, was it that Christ was making an important announcement like the typical Memorial service, that he was making an announcement like a politician at a press conference?
Or was he saying that the thief had been saved and he would be with him in paradise that same day?
That Christ's message remained to this day is because most people took notice of the second explanation. The first explanation is of benefit only to certain religious hierarchies.