Brett has raised an interesting question indeed. So, is compared a preposition? Or is it still a verb, which happens to share similar functions with prepositions?
I like the arguments presented by Brett; but I am not sure whether I am totally happy to say that compared is now a verb in one construction, now a preposition in another. Some lexical items are like that, I know, but this one?
I was thinking about this while taking a morning shower. My thoughts went like this -
The problem is that compare is a verb, pure and simple. Compared to ..., accordingly, is felt to be a participial construction. It is not like during as in during the summer; today, it is hard to imagine using dure as a verb.
Is it, then, perhaps more like the adjective interesting as in a very interesting book, which is no longer a participial form of the verb interest? Can you similarly say that compared as in Compared to ... is a preposition, no longer a participial form of the verb compare?
The problem here is that the criterion we rely on is the somewhat shaky notion of the 'understood' subject. It may be the subject of the main clause; it may be 'recoverable' from the linguistic context; or, more subtly, it may be 'interpretable' pragmatically. Right now, I am not sure whether the 'understood' subject is a grammatical property.
It is well known that coordination facts don't work well as syntactic arguments. Coordinated elements do not necessarily share a common grammatical category; they share a common function (today and on Wednesday).
So, can we perhaps say that compared shares a similar function with prepositions, say, that of clausal modifier? If we can, then we can say that a phrase headed by the past participial form of compare can modify a clause it is attached to, in the same way a phrase headed by a preposition can.
Then, if and when compared sufficiently loses its verbal properties, we will happily say 'Hi everyone, there was once a time when this was used as a verb, but now it is a preposition.' - and everyone will agree. Peace and happiness.
My shower was not long enough to pursue this further, and now I have to run; I have a second-hand book festival to go to. The beauty of a blog-post is I can leave it here, and invite comments from everyone - whom I wish a very happy Sunday.