I would like to draw attention to a recent article in Science “Why the Cosmological Constant is small and positive” by Steinhardt and Turok.
http://www.sciencemag.org/cgi/rapidpdf/1126231v1.pdf

I would be very interested to know what people think about this. Essentially, as I understand the argument, they propose that the cosmological constant starts big and relaxes through a number of transition to a low positive value. This however takes exponentially longer than the life of the universe, so Steinhardt and Turok calculate what happens if the universe cycles from big bang to big crunch over and over again. The Cosmological constant drops exponentially and for most of the existence of these repetitive universes it is small. Thus neutrons, atoms, and stars come into being, and life becomes possible. The argument in detail is beyond me, but it provides a possible way out of the contentious Anthropic Principle, since according to Steinhardt and Turok the cosmological constant is smaller than expected, (and so life exists) because the universe is exponentially older than expected.

There is a very good non-specialist commentary http://www.sciencemag.org/cgi/rapidpdf/1128570v1.pdf
Which helped a lot

Andrew