It seems that Nikon D5100 is quite hot item. Amazon is pretty thin on stock right now and appears to only have availability through third-party vendors at the moment.
B&H Photo has updated its stock status to show the D5100 as “Available for Pre-Order” which generally means that B&H has stock on the way and will be shipping it out in a matter of days. Here’s the link to the D5100 product page at B&H Photo.