found this on google:
 Some one should tell them that Live Long and Prosper is not a Star Trek original. In fact it was not even part of the Star Trek script, rather it was an adlib by Leonard Nimoy [Mr. Spock]. Nimoy has been a devout Jew his entire life and Live Long and Prosper is a bennediction that Rabbi's used to give over their congregation. In addition, the accompanying hand jesture is representative of the Hebrew letter Shin, which begins the word Shadai, one of God's sacred names. Just thought you might want to know the true origin of the phrase.
Steve