thejiro - Those plants look to be doing great! Congrats on the growth so far!
As far as your harvest question goes, that was my quandry last year. My best descriptors is when they feel like a ball of tissue paper. Light, papery and will spring back. Also, if you tear one open, you'll see lots of yellow/gold lupulin.
Here is a quick phone video I made and sent to a friend and harvested the next day:
johnnyboy, I'll never claim to be an expert but I disagree. I got about 11oz from two first-year Cascades last year and they are going absolutely insane this year.
2010 just before harvest: