The definitions of "Christian" are many and varied. Those who claim Mormons are not Christians are often correct, as long as their definition is the standard by which Christianity is measured. These arguments tend to latch on to Mormon beliefs that differ from most other large sects that consider themselves Christian. Specifically, they cite Mormon doctrines of baptism for the dead, temple ordinances, man's potential to become a God, the idea that Jesus is the spiritual brother of all, including Satan, alternatives to the Trinity doctrine, and others. If being a Christian requires a rejection of these beliefs, it is accurate to claim Mormons are not Christians.
Mormon apologists claim to be Christians based on alternative definitions. They tend to claim that being a Christian simply means that one believes in Jesus Christ as the Son of God and Savior, the only way to salvation and eternal life, and tries to the best of his or her ability to follow His teachings. If this is the definition by which one is judged as Christian or not, it is accurate to claim that to the extent they live up to these claims, Mormons are Christians.
If this is all relative, is there an absolutely correct definition? Perhaps not, but only one definition really matters in the long run, and that is the definition God Himself lives by. Rather than asking a religious scholar, a written creed, an online news story, or an opinion poll, the One to ask is Him! If He says Mormons are or are not Christian, what greater evidence can there be?