Sean, these are all great mock-ups. They take advantage of many design elements that users already know from sites like Flickr and YouTube. Of all the mockups, I prefer design A for a few reasons: 1) the browse by options are more noticeable at the top of the screen and don't span the entire page, which sort of distracts from the search box. 2) The collection info box at right provides nice context, whereas placing in in the banner as in D creates some confusion about what you're searching and browsing. I also like the distinction between the primary collection and the "also part of..." collection in A.

Some design elements I like in C,D:
C) the "item info" link in the action bar jumps to metadata if users don't initially notice the metadata below the fold. Also, there is a link to "comments and feedback" near the top, so users don't have to scroll to provide feedback.
D) the Google translator appears at the top of the page.

Some general comments/questions:
1) Will the metadata be linked in the Item Description section?
2) How will unusually shaped items affect item page layout? I'm thinking of long and narrow newspaper clippings, negative strips, etc.)
3) How will we display multi-page items?
4) How can we display full-text of an item, or long descriptions?

Again, these mockups are all great. Thanks for all of your work.