I found this article that concludes with this:
Vatican Library, Reference Service. "Re: Fwd: Verifying Information." E-mail to Marno Retief. 2 June 2004.
'It is, of course, a huge mistake. With much pain and time we found the passage you are quoting in the original manuscripts (Vaticanus latinus 2583, f. 258 v; Vat. lat. 1404, f. 22 r, both from 14th century), and in both it is clearly said "Dominum nostrum Papam". The wrong formulation, "Dominum Deum nostrum Papam", we found in an edition of the end of the 16th century, but these old editions cannot be philologically trusted. The original manuscripts have the correct version, and there is no word "Deum" in that sentence.'
Comments from Mr Marno Retief (who requested the information about the original manuscripts from Vatican Library)
'In Zenzelinus de Cassanis' original manuscripts (reference: Vaticanus latinus 2583, f. 258 v, and Vaticanus latinus 1404, f. 22 r, both from the 14th century) the word 'Deum' does NOT occur in the passage that anti-Catholics so often love to cite.' 'I am glad and thankful Vatican Library Reference Service so graciously assisted in finding out this information. The staff at the Vatican Library Reference Service was kind enough to go and look at the original 14th century manuscripts, and, as you know by now, they read 'Dominum nostrum Papam' and NOT 'Dominum Deum nostrum Papam'. Zenzelinus de Cassanis has been vindicated. For those anti-Catholics who are interested, there is an excellent article by Prof. Franz Gillman on how the erroneous phrase came to print in some of the later publications: Franz Gillmann, "Dominus Deus noster Papa"? (in: Archiv f. Kathol. Kirchenrecht 95, 1915, 266-282).'
http://www.angelfire.com/ms/seanie/forgeries/zenzelinus.html
It does seem a little over the top that anyone would refer to the pope as a deity.