Personally, I'm convinced that Shem and Melchizedek are one and the same. And I agree with the following Ensign explanation of D&C 84 posted earlier, especially if the verse in question is read with the two following it:
"On the other hand, there is a case for their being two distinct personalities. Many persons believe D&C 84:14 is proof that there are perhaps several generations between Melchizedek and Noah. The scripture says, 'Which Abraham received the priesthood from Melchizedek, who received it through the lineage of his fathers, even till Noah.'
"If it does turn out that Shem and Melchizedek are the same person, this scripture should prove no stumbling block, because it could be interpreted to mean that priesthood authority commenced with Adam and came through the fathers, even till Noah, and then to Shem."
Also, one of the more obscure definitions of "even" is "or more exactly" (taken from the Encarta Dictionary via Microsoft Word). Hence, the passages could possibly be reworded as "Melchizedek received it through the lineage of his fathers, or more exactly, from Noah to his fathers, including Enoch, Abel, etc," or maybe, "through the lineage of his fathers, starting at Noah..." These interpretations seem more plausible also when you compare verses 14-16 with the more precisely worded verses 6-12.
"If the Church wasn't true, the missionaries would have destroyed it years ago..." -J. Golden Kimball (or at least attributed to him)
"Don't you kids know anything!? The Serpent of Rehoboam? The Well of Zohaph-zadar? The Bridal Feast of Beth-jebuharazeb?" -Ned Flanders