Mello--Mihael Keehl
Everyone's opinion is mixed on what Mello's real name means in regards to his ethnic background; on the forums I've heard everything from his name being German to Scandinavian to Russian to It's Just Made Up.

The closest thing in English to Mihael is, obviously, Michael. The name meaning, roughly, is "one who is like God," which could be taken several ways. Near once tells Light that Mello might want the Death Note to take over the world (becoming God), though this is proven incorrect. If L is put symbolically as "God," Mello certainly fits the bill, with his very high intelligence and daring. Also, Mello did want to become "God" in a different way, showcased towards the very end of the series-- by surpassing L.

In the Book of Revelations in the New Testament, Michael the Archangel is mentioned as having fought with his angels against the dragon (the Devil) during the war in heaven. If L is "God," Light can easily be taken as the "Devil," making Mello's real name fit even better.

"Keehl" is also phonetically similar to the word "kill" and could be a reference to Mello's status as a murderer.

Matt--... Matt?
It's never outright said whether or not Matt went by his real name or not, but considering it's not a brand of soda, a letter of the alphabet, or a noun, Matt stands a fair chance of at least going by something close to his real name.

Some fans believe that Matt is an American; however, this is pure speculation.

Matt generally is short for Matthew, which translates into "gift of God." If L is the symbolic God, Matt, coming from nowhere with just a background at Wammy's to help Mello, is fulfilling his name. Biblically, Matthew was a tax collector (also author of the Gospel of Matthew), but past that little is very certain about his life. Alternately, and much less likely, Matt could be short for the name Matthias, which means "gift of Jehovah," and was the name of the man picked to replace Judas Iscariot in the Twelve Apostles. Tradition holds that he ended up martyred in Ethiopia.

Near--Nate River
People seem to think that Near's an American, judging from the last name, but nothing was ever stated for certain.

Biblically, Nate is close to either Nathan or Nathanael. Nathan translates into "He has given," and in the Old Testament, Nathan told David not to build a temple, told him off for getting Bathsheba's husband killed so he could marry her, and anointed Solomon as king.

Nathanael translates into "God has given," and, again, if L is God, symbolically, Near's name fits. Near is smarter than Mello, and, had L lived to choose a successor (and had he chosen based only on intelligence), Near would probably been picked. Mello and Near both got the case, but with Near's name, it could be taken that the case was really given/sanctioned for him. And after L and Mello ("one who is like L/God") failed, Near ("L/God has given") is the one that defeats Kira. In the Bible, Nathanael, weirdly enough, is suspected to be the same person as Bartholomew and is an apostle. Traditionally, Bartholomew was flayed alive and then crucified upside-down.

L--And Theories
No way are we ever finding out L's real name now-- and really, it could be anything, but if you want to keep with the Biblically-derived names/names starting with his alias, Luke is a decent guess (and would be a blatant hint to a non-Japanese background).

Luke means "bringer of light," a double meaning (light into the truth involving the Kira case, but also Light as in Light Yagami). Luke wrote the Gospel of Luke and the Book of Acts, and was a physician who traveled with Paul. He was imprisoned several times in Rome, and tradition says he died a martyr.

Thank you to my Bible Dictionary and Bible for the name translations and most of the basic information, and Wikipedia for additional information on the martyrdoms, etc. Thanks also to Sister Acker, who taught my Seminary class.