Good quick review of available tools to do Life Cycle Assessments: http://www.linkcycle.com/comparison-of-best-life-cycle-assessment-software/ and here
I like the way Linkcycle thinks – focusing on the most strategic and biggest influencers on lifecycle impacts, rather than doing monster assessments. They even offer a free quick review tool.
Of course, the biggest challenge of LCA is comparing apples to apples based on the best data applicable to your products and processes.
11/20/12 additional info: Interesting review of Berkeley LMAS study comparing results of different LCA models. It really matters what assumptions your model uses! Paper cup rankings for 3 different models: 1, 3 and 6/6 !