Several references note both "devil" and "Devil."
For example, the American Heritage Dictionary*'s entry for "devil" is this:
"¢ often Devil In many religions, the major personified spirit of evil, ruler of Hell, and foe of God. Used with the.
The LDOCE**'s entry:
"¢ the devil also the Devil
The Collins COBUILD***'s entry:
"¢ In Judaism, Christianity, and Islam, the Devil is the most powerful and important evil spirit... There are two forces at work: God and the Devil.
"¢ 2. a devil is an evil spirit...the idea of angels with wings and devils with horns and hoofs.
The Collins COBUILD has 11 entries for "devil," all without a capital. The references to sayings all have "devil" in lower case: "better the devil you know than the devil you don't know....had a devil of a job....like th devil....the devil take the hindmost...between the devil and the deep blue sea...speak of the devil...what the devil...."
The New York Times Manual**** states:
"¢ devil. Capitalize it in references to Satan, but lowercase devils and a devil.
The Oxford Dictionary of Allusions***** contains two pages of alternate names for "the Devil," and describes "Devil" this way:
"¢ "In Christian and Jewish belief, the Devil is the supreme spirit of evel. He is the enemy of God...The Devil is known by numerous names, especially Satan and Lucifer..."
From the information above, it appears that "devil" is capitalized when it is a personification, when it is the name of a being. "Devil" is not capitalized when it refers to a spirit or the idea of evil.
Sometimes the distinction between a personification and a symbol are blurred, however.
*The American Heritage Dictionary of the English Language, Fourth Edition. Houghton Mifflin. 2004
**The Longman Dictionary of Contemporary English. New Edition. Longman. 2003
***The Collins COBUILD Dictionary of the English Language. Harper Collins. 1995
****The New York Times Manural of Style and Usage, by Allan M. Siegal and William G. Connolly. Random House. 1999
*****The Oxford Dictionary of Allusions. Oxford University Press. 2001