I understand that the meat is thinned with a mallet before coating with breadcrumbs and frying.
What parts of veal or pork are suitable?
(Googling the subject seems to produce mostly sausage related hits for some reason)
The real, original Wiener Schnitzel is veal. Definitely. I can't link to a web source now, but I have some German cook books at home, including one solely about different kinds of schnitzel.
From the veal, you use parts 6, 7 and 11 for any kind of schnitzel, 7 being the best and highest-prized choice. I don't know the English names, maybe somebody can supply them. Also, try using leaner meat. I think that in the US, the fat marbled beef is considered higher quality. But in Europe, lean beef is preferred. A good beef schnitzel has maybe 3% fat.
This said, there are lots of places which cook pork schnitzels in the Wiener style and call them Wiener Schnitzel. It isn't traditional, but if you like the taste, there is nothing against doing it that way at home. They come from the same parts of the pig as the veal schnitzel: the ham and the lower back. Again, they should be very lean. And in both cases, unlike a steak, a schnitzel is always boneless.
German Food Guide says to use the pork loin.
Cooks.com says to use either a pork chop or a veal cutlet.
I was taught that the "Wiener" referred to the original recipe origins in Vienna. This Schnitzel in the Vienna style was originally veal.
American German restaurants usually serve a variety of meats prepared into paillards (flattened with mallet) and breaded - generally only the Veal one is called "Wiener Schnitzel".
At home, I prefer to use Center Cut pork chops to prepare what I like to call "Swiner Schnitzel". I agree with the answer above that leaner is better. Schnitzel cooks extremely quickly and marbling is unnecessary.