Under what conditions does the Bible support divorce?

It seems that Jesus talks out against divorce. But in other places, divorce is allowed. How does this all play out and where does the bible really stand on this? Is it a sin or is it just part of the culture? I'm seeking the exegesis of [mainstream Protestant](http://en.wikipedia.org/wiki/Mainline_Protestant) denominations, since I believe that these will all be similar (if not identical). If differences of interpretation exist between the different denominations, please let me know and I'll refine my scope a bit.