We are however, certain of a few things; namely that he is now in heaven because Jesus told him so and that Christ did not establish the necessity of Baptism till after His ressurection.
Since Christ did not institute the Sacrament of Baptism until after His ressurection, whether or not the good thief was actually baptised or not is of no consequence.
***Since Christ did not institute the Sacrament of Baptism until after His ressurection...***
I find this difficult to believe in light of the following passage...
"Now when Jesus learned that the Pharisees had heard that Jesus was making and baptizing more disciples than John (although Jesus himself did not baptize, but only his disciples), He left Judea and departed again for Galilee."
-John 4
It is clear from the above that Jesus' disciples were baptizing quite early in his ministry. Now unless you think they were doing this of their own accord, it is fair to assume they were doing it with Jesus blessing.
Also...
If baptism is so critically important in terms of salvation and spiritual new birth, why do we find Paul with such an unconcerned attitude towards it?
"I thank God that I baptized none of you except Crispus and Gaius, so that no one may say that you were baptized in my name. (I did baptize also the household of Stephanas. Beyond that, I do not know whether I baptized anyone else.) For Christ did not send me to baptize but to preach the gospel, and not with words of eloquent wisdom, lest the cross of Christ be emptied of its power.
- 1st Cor 1
Spiritual birth must be accomplished without the necessity of baptism for Paul, in the same epistle, to the same readers, claims...
"I write not these things to shame you, but as my beloved sons I warn you. For though ye have ten thousand instructers in Christ, yet have ye not many fathers: for in Christ Jesus I have begotten you through the gospel."
- 1st Cor 4
In other words, through Paul's preaching of the gospel to them something happened that was powerful enough to transfer them into the family of God (i.e. new birth) - and this without then necessity of baptism as seen in the first chapter.