The pitch in the 17th century was not standardized. Thus, a’ kammerton would differ from country to country, or even between cities in the same country. Such was the case in Italy as well, where the pitch was “moving” up or down as much as by a minor third.
How do we know which pitch was used in the music by Monteverdi? We know this from surviving tuning forks, wind instruments (such as cornetto), or organ pipes, which were made with a fixed pitch. We know that the pitch in Rome was, therefore, around 392 Hz (equal to today’s g’), and in Venice as high as 466 Hz (equal to today’s b’ flat).
The same piece could be played at a variety of different pitch levels, which (if you believe anything about the characteristics of keys) would change the character (temperament?) of the piece greatly. And here is Gosta's review of the recording to be aired by WGBH:
Martin Pearlman is known in the North-American community as a notable presenter of historically informed performances. With his Boston-based ensemble, he presents a recording which follows traditional theories about Monteverdi’s Vespers, but with a slight “romantic” approach. The singers are decent, but less experienced in early music style than on the Parrott’s and Pickett’s recordings. His tempi and division of sections are unconvincing and too vague for an experienced early musician. However, this recording is a pioneer in Northern American learning early music community.
The last sentence doesn't make any sense, but overall Gosta recommends Andrew Parrott's recording with the Taverner Consort, Choir & Players. However, I plan to listen to the webcast, since I haven't had a chance to hear Pearlman's version and I do like his recordings of the Brandenburg Concerti. The Vespers will be aired in parts at 10 am on each day in Cathy Fuller's Classics in the Morning show.