Very interesting question. I did some browsing and came up with the following information.
First of all, this link has some content on the topic:
http://answers.google.com/answers/threadview/id/271624.html
Here is one quote from that thread, but I recommend reading it and judging the content for yourself:
The Talmud relates how the Babylonian Sage, Rabba (Abba ben
Joseph, C. 280-352), used to pray with his hands folded.
So "folding hands" could have come down from the Jewish heritage of Christians. Others also distinguish between "praying hands" and "folding hands".
And for the meaning of it, I came across the following web page:
http://www.stjosephsite.com/SJS_Articles_prayinghands.htm
It mentions the following information (pasted in fragments):
Such a prayer gesture is a symbol of obedience, submission, sincerity and repentance.
...
Nathan Ausubel, in his book "Book of Jewish Knowledge said that this prayer gesture is not exclusive to Christians alone. He said the Jews even before the time of Jesus Christ, practiced it: "It has also been commonly assumed that folding the hands in prayer is exclusively a Christian custom. This is not the historical fact at all. As early as the post-Exilic period, when Jews prayed, they folded their hands, and they observed this custom for several centuries even after it had been adopted by Christians."
Jesus, being a Jew, also prayed with clasped hands. You must have seen pictures or images of Him in the garden of Gethsemani in this prayer gesture. In other art works, you can also see the Virgin Mary and angels with their hands clasped as well.
...
The Pope mentioned the custom during feudal times of placing one's joined hands into the hands of his ruling lord as a sign of his fidelity and loyalty. So when we fold our hands in prayer, we are symbolically pledging to God our fidelity and loyalty and placing our hands in His.
And another viewpoint here:
http://www.trivia-library.com/a/why-does-a-person-join-his-hands-when-praying.htm
The joining of hands "leads back to men's early desire to subjugate each other and developed out of the shackling of hands of prisoners!
"Though the handcuffs eventually disappeared, the joining of hands remained as a symbol of man's servitude and submission and his inability (or even lack of inclination) to grasp a weapon."
Christianity adopted "the gesture representing shackled hands as a sign of man's total obedience to divine power."
Another reference (and a selective quote from it):
http://www.answerbag.com/q_view/421928
"Religious historians trace the gesture back to the act of shackling a prisoner’s hands with vine or rope: joined hands came to symbolize submission. In ancient Rome, a captured soldier could avoid immediate death by joining the hands together. Just as waving a white flag today, the message was clear. “I surrender.”
Centuries later, subjects demonstrated their loyalty and paid homage to their rulers by joining their hands. In time, clasping the hands together communicated both an acknowledgement of another’s authority and one’s own submission to that authority."
Books by Nathan Ausubel:
http://www.amazon.com/s/ref=ntt_athr_dp_sr_1?_encoding=UTF8&sort=relevancerank&search-alias=books&field-author=Nathan%20Ausubel
