Presumably, the immorality of chattel slavery represents one of many transcendent, objective, moral truths, the likes of which emanate from the will or nature of God.
What then, does God have to say about slavery? I've been meaning to write a blog post about this for some time, but then I stumbled upon this one and I really don't think that I could do a better job considering how well linked this one is to relevant Bible passages.
The only thing I would add is that the idea that the slavery instructed in the Bible only represents "indentured servitude" (where someone serves another as a debt and only until the debt is repaid) is just false. While the Bible does instruct indentured servitude of some Israelite slaves, it also endorses chattel slavery (life-long, permanent ownership of slaves and their offspring) and sexual slavery (Exodus 21:7-11) for non-Israelites. This is the most despicable type of slavery imaginable, and Leviticus 25:44-46, well, let me just let NonStampCollector make the point as only he can:
So without further ado, here's someone only known to me as "The Beagle". You can follow the active link over to his blog for other interesting reads.