Whats up with the acorns?

I have only ever seen these:
http://www.flickr.com/photos/silvia31163/213161941/
And now all I seem to find on blogs are these:

http://www.flickr.com/photos/disdatmac/1912443720/
As far as I know all these blogs are in the States. is this an oak only found there? I love the way their acorncaps look....