Pottery Barn has a new Dr. Seuss line - totally cute and unisex. If you don't want to go broke buying the whole line, you could just get the crib sheet and mix/match it with matching solid bumpers and crib skirt(if you use them). Finally, you might be able to do a google search for Dr. Seuss wallpaper border, place it about chair rail height, leave the top part of the walls white, and paint the bottom parts of the walls in red, yellow, blue, green. On the top parts of the walls, you could stencil quotes or lines from his books.
I'm not sure what all is out there, but google for throw pillows, mobiles, curtains (or just get cheaper, solid ones to match). You could paint wood letters in the baby's initials in primary colors as well.
I would love to do this theme, but if we have another boy we are going to geek it up super huge and do a Star Wars theme at dad's request and if it is a girl then we're going with a pink and green motif as a nod to my sorority colors.
Currently our son has a University of Illinois nursery and it is awesome!