I was able to compare the viewfinders of the Pentax K10D and Nikon D80 today, and discovered that they are exactly the same size.
Posted by: MIKE JOHNSTON
ADDENDUM: I didn't shoot with these cameras or do any sort of detailed comparison, and am not prepared to write a full review of either of them, separately or together. Although I agree it would be an interesting article.
One thing I can say is that the D80 feels markedly more comfortable in my hand and held to my eye, although I could certainly live with the Pentax. This is not a judgment of either camera, really; it's simply personal preference, and you might feel the same or the opposite. It does underscore the need to "heft and hold" a camera yourself before making up your mind about it. When the Nikon D70 and Canon XT were current, I knew people who strongly preferred the ergonomics and hand-feel of one, and other people who felt the same way about the other. It's still an important consideration for most people even though it's completely subjective.
ADDENDUM 2: Oh, and for what it's worth, the salesman was concerned when I asked to see the K10D and the D80 together, apparently fearing that the D80 would suffer in comparison. He suggested that the D200 was a more appropriate comparison for the K10D, and pulled one out and set it on the counter for me without being asked.