I have more to come, but I thought I'd post this real quick. Our kale from last year has begun to flower and go to seed. I thought it would be beneficial to someone if I posted what it looks like.
The old kale plants (and other plants like it) all the sudden start growing taller and taller and the stock gets really thick. Shoots start growing out of the top and then flowers come (kale flowers are yellow). Eventually the flowers and plants die and seeds are ready. I have not seen the kale get to this stage yet, but I remember our lettuce and spinach at this stage last year and the seeds were much like dandelion seeds (you know when the head of the dandelion flower is all fuzzy, you blow it and all the seeds come off?).
UPDATE: The kale plant starts to produce seed pods. The pods get bigger and bigger and eventually (as the plant begins to die) the seeds will be ready to harvest.
The bed of old kale going to seed:
A more artistic view of the flowers (thanks to Brianna):
The thick stock:
I'll update this post with pictures of the kale seeds whenever I can.
UPDATE (5/23/12) - new pictures!
This first picture is of the new kale. We use kale in green smoothies and also chopped up small in soup or with tomatoes over rice. A huge bowl of kale cooks down to nearly nothing, so we can put a lot of kale in a main dish. I've heard that kale is the most nutritious dark green leafy veggie you can grow in your garden and (as we know from experience) it's VERY easy to grow. We plan to plant more of it soon.
See how tall the seeding kale from last year has gotten? The yellow flowers are mostly gone and there are hundreds or thousands of seed pods.
A close up of the seed pod. I think they will be ready to harvest when the plant dies and the pods start to open.
UPDATE (5-28-12) New Pictures!
The seed pods are nice and dry now and some have begun to burst open. In the last picture of the seed pods, you can see that they are still a bit green. Now however they are a light sandy color and are dry. Here are a few pictures of the dry pods as well as harvested seeds. We have thousands of them. Many thousands. We'll have to share :)
A final note about saving kale seeds ... collards, broccoli, Brussels' sprouts, cauliflower, cabbage, kohlrabi, and kale, except Siberian kale, are all in the species Brassica oleracea, so they will all cross pollinate. When plants of the same species cross pollinate, their seeds will no longer be pure and may or may not produce anything next year. If they do produce next year, the results may or may not be the vegetable you were hoping to get and then the seeds from the year after that will not work!
You have a few options to prevent cross pollination from happening. One, you could just plant one plant from the same species. Two, you can plant what you want but pull up the others of the same species before they start flowering and only let the one go to seed that you want to save. Three, plant the same species in different gardens, very far apart from each other. (this method is not fool-proof! you'll have to do some investigating to see how far apart is recommended. sometimes it's 100 feet. sometimes it's much further.) Four, protect different plants from the same species from cross pollinating. For example, C. maxima (squashes like hubbard, buttercup and some varieties of large pumpkins) -- you must purposefully pick a male flower and use that to pollinate a female flower; this is called hand pollination. The female flower must remain taped shut so there is no danger of cross-pollination with another of the same (or similar) species. Corban then puts a piece of tape around the vine that has the properly pollinated fruit on it so he knows which ones to save seeds from.
But this post is about kale so forget about option 4! It seems the safest thing to do for brassica oleracea would be to plan on saving seeds from, let's say, kale in the spring/summer and broccoli in the fall/winter. Option 1 or 2. :)