Difference between ではない and でない

Short answer: they can be used interchangeably.ではない is more commonly used.

は has strong affinity for negative expressions, in other words the function of は referring to topic is almost neutralised, or at least weakened in negative sentences.

でない is just a negative, e.g. He's not Christian.
ではない is also negative, but implies something else. e.g. He's not Christian (but Buddhist (say)).

e.g. 私はりんごが好きではないが、（みかんは好きだ。）
私はりんごが好きでない。

Source: http://answers.yahoo.com/question/index?qid=20080328040511AARlO5j