Steves's last post, and his paraphrasing of Mr. Park's book, seem to confirm my recollection of the story of the plans at the Mariners' Museum - with the big proviso that my information was about 25 years out of date. Sometime after I left, I gather, the museum came to terms with the gentleman in Norfolk and bought at least some of the plans he had. (My guess is that nobody was willing to pay the price he was asking, so he dropped it. Or maybe that particular individual died, and whoever inherited the drawings was more reasonable - or needed the money. Maybe those other members of the Porter family that steves mentioned were in that category.) I think steves and Mr. Park are right: for such a repository to acquire those documents was certainly a proper thing to do, but the documents themselves did not, by definition, add anything significant to the extant knowledge of the ship. I'm pretty sure the MM had photocopies of them as early as the 1930s - and the gift shop was selling copies of the copies when I was working there, in the early 1980s.
I got some first-hand experience with the relationship between Confederate ironclad plans and reality a few years back, when some of my students and I were working on a project involving the C.S.S. Neuse. (The remains of her hull are preserved at Kinston, NC, about 35 miles from where I live. She's in a less-than-ideal state at the moment, but there are big plans afoot for her future.) There's a set of plans for her, by the same Mr. Porter who worked on the design of the Virginia. Those plans supposedly were used to build the Neuse and the Albemarle, which supposedly were identical (or near-identical) sister-ships. Some photos of the Albemarle have been found; they make it clear that (a) she was quite a bit different from the Neuse (though the two ships had similar dimensions and basic shapes), and (b) there were plenty of differences between the Albemarle and the plans. I drew a new set of four views of the Neuse, based on the best information the guys at the state historic site and I could put together; I ran into all sorts of little problems that made me question just how reliable those old Porter drawings were. (We had a good deal of trouble, for instance, reconciling the written documentation about the iron shutters over the gunports with the Porter plans - and the photos of the Albemarle. I'm not at all confident that I got those details right.) I eventually came to the conclusion that the Porter plans were intended as a "contract draft," a highly generalized set of specifications that constituted part of the contract between the Confederate Navy and the various people who actually built the ship, rather than a precise guide to how the ship was going to look. It's pretty clear that the actual work was carried out by people who had little, if any, experience with shipbuilding; they built something that worked (well, more or less) and met the requirements of the contract. Whether the finished ship looked like the drawing probably didn't matter much.
It's perfectly normal, even in much more modern times, for significant differences to appear between the plans that are drawn in advance of a ship's construction and the ship herself. I stongly suspect that was the case with the Virginia. The conversion from steam frigate to ironclad ram was done in a great hurry, and nobody involved had ever done such a thing before. It seems likely that a great deal of improvising took place - and adhering closely to a set of plans probably wasn't a high priority.
I haven't seen the article Al Ross mentioned, but the idea of a faceted casemate certainly is believable. Logic suggests that a wood and iron structure composed of flat surfaces is easier to build than one with curves in it - especially for people who are in a hurry. I'm no expert on Confederate ironclads, but my recollection is that none of the others about which any documentation or photos have survived had any curved surfaces on their casemates. On the other hand, the Virginia was the first of the batch, and the biggest; it wouldn't be surprising if she turned out to be unique in that respect as in so many others.
It sure would be nice if somebody would find some photos of the Virginia, and lay all this speculation to rest once and for all. But we shouldn't hold our breath. The photographic record of the Civil War, though enormous, has quite a few remarkable gaps in it. (For instance, when my students and I were working on that project in Kinston we spent a good deal of time digging in various excellent sources, and consulting with qualified people, in search of photographs of Confederate Navy enlisted men. We never found any.) If any photos of the Virginia ever do turn up, my guess is that they'll show some things that surprise everybody.