I had a Latin/Roman professor who told my class once that Mercury wasn't a native Italian god, just the Greek Hermes given a different name. I don't know any primary sources to back that up, but I have no doubt he knew what he was talking about.
I'm obviously waaay late for this topic, but if it's still pertinent and since NibbleKat asked for more academic data, I may be able to add two cents.
From what I've read, one of the reasons why scholars say that Mercury is Hermes renamed is that He had no flamen. It was one of the most ancient priesthoods in Rome and it can be a good indicator of how old a given cult was. Jupiter, Pomona and Quirinus, for instance, all had flamens; Mercury did not and that could mean that He was not worshipped since earliest times. Of course, it could simply indicate that He was never assigned a traditional priest, just like there's no record of Janus having a flamen and even though He is a very old Roman god.
There's also the only known Roman temple to Mercury, supposedly located on a slope of the Aventine Hill. According to tradition, it was dedicated in 495 BC and was outside the
pomerium, i.e. the city's religious boundary. That was sometimes - or even usually - the case with imported deities: the temple of Isis, for instance, was originally built on the Field of Mars, which was outside the
pomerium; Apollo's too was originally outside of it. There is, however, the case of the Dioscouri, who are clearly Greek in origin, but were given a temple in the heart of Rome. Yet They helped the city in battle, so I guess that made Them "adopted Romans" of sorts. Interestingly, Apollo received a temple inside the
pomerium only after Octavian Augustus' victory at Actium, which he attributed to the god's intervention.
One last piece of evidence: if the date of a temple's dedication is indicative of when the cult was introduced in Rome - which may well be the case of Mercury's - then it occurred around the same time as the Dioscouri's, whose temple is said to have been inaugurated in 484 BC; also Apollo's in 431 BC. These are notorious cases of Greek influence over Roman religion. If one considers how close Magna Greece was and the commercial and cultural ties that may have existed between Rome and the Hellenic colonies in southern Italy... well, it's not a long stretch of imagination to see how Hermes may have been imported by traders in the same period as other Greek gods were.
The name is different, sure, and the same may be true for cultic elements. But Rome's Apollo was a soft version of Greece's, which is what usually happens when a deity is imported: it's reinterpreted according to the needs and culture of the host society. So if Hermes gained devotees among Rome's merchants, that would naturally mould his cult in the city, including His name: Mercurius, from Latin
mercatura (merchandise) or
mercatus (market).
If I may end with a bit of UPG: as a Mercury devotee, I can say that I see little difference between Him and Hermes. Or at least Hermes seems to respond to the Roman name. Maybe there’s an element of cultural awareness that moulds His actions with me, much like I’d behave differently in an Arab country if I happened to visit it often and came to be known by an Arab nickname. People who travel a lot are sometimes known through different names in different countries and will respond to the cultural praxis of each. The same may hold true for gods of travellers and especially if They’re as versatile as Hermes appears to be. And if some say that luck is more of an hermetic than mercurial element, allow me to share my first (conscious) experience with Mercury: I found 5 Euros on a floor of a busy place, bought a lottery ticket with it and won 25 Euros. And this happened during the week when I got my caduceus pendant in the mail.
Hope this helps.
In Pax Deorum,
Helio