Wood blocks are the best, especially the ones from miller goodman. They're just the thing for creative little minds.
My favorite kid blog is ohdeedoh. It's time for their annual Small Kids, Big Color contest, which is a great chance to see and vote on some beautifully designed spaces. Here are two different blue grey nurseries that are in the running. You'll have to check the blog for more pictures.
I'm not usually a fan of wall decals, but the designs from haciendo el indio are a sweet exception.
Check out klt works for their cool mobiles, letterpress prints, t-shirts, and stuffed animals. KLT stands for Kristin Loffer Theiss, and this is the nursery she created for her son: