God made the human body a marvelous mechanism for protecting us against external hazards, and we violate His handiwork at our grave peril. Some top Google links on the topic:
 

The American Dental Association (ADA) recommends that its members discourage patients from having oral piercing by educating patients about the health risks. (Dr.Dave and Dr.Dee)

Tattooed artwork involves breaching one of your body's main protective barriersÇ -- the skin. (Mayo Clinic)

The risk of infection is the reason the American Association of Blood Banks requires a one-year wait between getting a tattoo and donating blood. (Medicine.Net)
 

Do not cut your bodies for the dead or put tattoo marks on yourselves. I am the Lord. (Leviticus 19:28)

This verse is in the same context, and has the same form as the Second Great Commandment. It is curious that several pro-tattoo sites refer to this verse and go on to point out,
 

Tattooing has been discouraged or forbidden by most Christian churches throughout history. (Tattoos.com)

A hundred years ago, and you would have been very hard pressed to find anyone wearing a Christian tattoo design. As a part of history, tattoos have been most often frowned upon by not only the Christian clergy, but also the Jewish and Muslim faiths. (TattooJohnny.com)
 

It has been argued that a good motive or using Christian symbols in the tattoo sanctifies the practice. I suspect the thinking is similar to the idea of making money by immoral means in order to put a big check in the offering plate. The Law of Moses itself forbids that [Deut.23:18]. Jesus also demolished that argument in Mark 7:11, where the Pharisees were declaring as "Corban" (devoted to God) the resources required by the Mosaic Law to be used other purposes. However virtuous our goals (be they Christian symbols or a check in the offering plate), they do not and cannot nullify the Word of God.

If I am to submit myself to the authority of God, I cannot allow myself to pick and choose which verses to obey and which to ignore. Scriptures clearly teach that the Mosaic sacrificial system was for the Jews, and has been done away with in Christ. There are no such teachings about the principles in Leviticus 19; moreover, several of those commandments are repeated and specifically addressed to Christians (v.26 in Acts 15:20, v.18 in Matt.22:39, v.32 in 1Tim.5:1, etc).

Some other relevant Scriptures:
 

He [the master] shall take him to the door or the doorpost and pierce his ear with an awl. Then he will be his servant for life. (Ex.21:6)

This verse describes the pierced ear as a symbol of lifetime servitude. While some Christians conscientiously do it to reflect their bondage to Jesus Christ, there is a significant difference. In the Law of Moses, it's the master who does the piercing; the slave does not do it to himself. Jesus himself taught:
 

If the Son sets you free, you will be free indeed. (John 8:36)

Along the same line we have:
 

He was pierced for our transgressions... (Isa.53:5)

While Jesus volunteered for this piercing, it was not a positive thing to be taken lightly. At the very least, if we want to take up the cross of Jesus in this way, it should be for a specific benefit of other people (in Jesus' case, it was to take away -- actually, not merely symbolicly -- the sin of the world). Being martyred by arrows or bullets piercing one's body in the line of Christian duty might be an instance of this, but I can't see decorative piercings in the same category.

While it's not my place to criticize people who have already -- perhaps in a time of weakness or lesser spiritual maturity -- taken this irreversible step, I prefer to err on the side of safety before encouraging it or choosing to do it myself.

For additional thoughts on tattoos in connection with Lev.19:27 on cutting your hair, see Decorative Hair.

I recently found a link dealing specifically with the Hebrew of Lev.19:28.
 

Rev. 2008 March 1