I'm with Faldo on this one. One way of determining a part of speech (word class, lexical category) is by form. They're those words that end in -ly in English. Another way is by what slots they fill in sentences "I found the book ___". The former is the older grammatical tradition, and the latter is the newer one. If you limit adverbs by form, you get a bunch of exceptions like fast, yesterday, etc. The slot method just seems better suited for a language like English which is not overly burdened with inflections, such as Greek or Latin.
If, I say: "I found the gold hoard in situ", it is answering the question "where did you find the gold hoard", and that pretty much seems like an adverb to me. Those who prefer the the by form method of identification usually also have problems with constituents (sentential units) consisting of more than one word. For me, prepositional phrases can be adjectives or adverbs, or used adjectivally or adverbially.
