Here are links, first to a quick summary, then to a slightly more detailed story:
The herbalist's book theory has been raised before, the main twist this time being to shift the focus from the Old World to the New (though even that claim has some precedence; see D'Imperio pages 73-75 and 14-16).
Where this newest theory seems to break down is in the statement that
"names of various plants have been identified in Nahuatl".
The problem with this is that it's not just the language but the script that's unknown. That is, no one knows what sounds or significance the individual letters or glyphs have (or even how many letters there are, or whether they're letters or ideograms), which has made it impossible to identify the underlying language. So anyone who claims to have identified specific words in the Voynich Manuscript needs to explain the whole decipherment process by which he or she was able to read individual words within the manuscript.
In any case, it's amusing to note that this announcement (that the Ms dates from the early 1500s) flatly contradicts the last major 'discovery' regarding the Ms (the claim that radio-carbon dating established that it dated from the early 1400s).**
For a fascinating summary of theories regarding the Ms, see M. E. D'Imperio's THE VOYNICH MANUSCRIPT -- AN ELEGANT ENIGMA [n.d., circa 1976]
Unfortunately, there doesn't seem to be a facsimile edition available, but here's a wonderful site that shows scanned in images of every page, so you can see for yourself just what this weird and wonderful book looks like.
I still stick to my own theory: it's not just an invented alphabet but also an invented language -- as if we had a book in Sindarin written in tengwar, with no other information on either Sindarin or tengwar outside that single book. That's why I doubt the Voynich Manuscript will ever be deciphered, and I suspect we wdn't learn the author's name even if it was. In the meantime, it's a wonderful canvas for people to project their theories onto.
*with thanks to Janice for the link
**see my earlier blog post from February 2011: http://sacnoths.blogspot.com/2011/02/voynich-manuscript.html