I've seen an article that said that the average American has less than 3K in his retirement plan, another that said they have 15k, another just the other day that said 54K. Wonder what the average really is?

What I'd really like to know is what the average retirement plan has in it based on age. "The average 40 year old has $X in his 401K or IRA." Anyone seen this stat or a web site that tracks it?