A brief history, courtesy of The History Channel:
"One legend contends that Valentine was a priest who served during the third century in Rome. When Emperor Claudius II decided that single men made better soldiers than those with wives and families, he outlawed marriage for young men -- his crop of potential soldiers. Valentine, realizing the injustice of the decree, defied Claudius and continued to perform marriages for young lovers in secret. When Valentine's actions were discovered, Claudius ordered that he be put to death.
Other stories suggest that Valentine may have been killed for attempting to help Christians escape harsh Roman prisons where they were often beaten and tortured.
According to one legend, Valentine actually sent the first 'valentine' greeting himself. While in prison, it is believed that Valentine fell in love with a young girl -- who may have been his jailor's daughter -- who visited him during his confinement. Before his death, it is alleged that he wrote her a letter, which he signed 'From your Valentine,' an expression that is still in use today."
JT of DaBoys points out that the Catholic Church disavowed Valentine's day as a holiday in 1969. He uses this contention as proof that it is not a Christian holiday. I would argue the opposite: that it clearly was a Christian holiday, and therefore, no matter what it is considered now, it cannot be seperated from its Christian past. Ari Kinsberg of Ari's Blog doesn't celebrate the holiday, not because it's Christian, but because we have a holiday of romance, Tu b'Av.
I am not sure if Reverened Robbie of Today in Alternate History is a) an anti-semite, b) making things up whole cloth, or c) both, but his post on Valentine's day includes a "fact" that would be interesting if it were true. Basically that there was a proposed massacre of Jews by Christians which was only stopped when one woman claimed to have seen the saint. Precious Umdurman of Precious World records the same events, except that in her version 2,000 Jews were killed in 1349 (ostensibly for causing the plague), and there was no reprieve. Poison Pero of This Day in History has an account which agrees with that of Precious. Mmm, nothing gets me in the mood like a massacre.
And an interesting aside, Believer786, a convert to Islam posts about whether or not Muslims should celebrate Valentine's day. His answer is pretty clearly that they should not. But the reasoning makes for an interesting read.