Karl Barth supposedly once said that every sermon should be written with the Bible in one hand and a newspaper in the other. His point was, if in fact he said it, that preaching should help the hearer to be in dialog with scripture, one's self, and the culture in which one lives. Barth would, I suspect, be scandalized by how many Christian leaders who think they need to nip and tuck at scripture to make it more relevant to contemporary society. Scripture IS relevant to culture. However ancient the text may be, its commentary and critique of humanity in all its glory and all its foibles is evergreen. Fashions may change with the times but people don't. We moderns (or Postmoderns, if you like) struggle with the same problems, confront the same challenges, have the same needs and hopes and dreams as did those in the Bible's original audience. Worthy preachers would do well to demonstrate that truth to their congregations, regularly.

Owen Strachan has some thoughts on the subject, and has written an interesting blog post on the three reasons he thinks preachers should preach about the "public square." They are:

The church body needs wisdom to live in a fallen world.
The secular culture needs to hear from the true culture.
Believers have historically been change agents because of strong pulpits.

Strachan's prose is clear and cogent. The whole article is worth taking the time to read and consider.