There's a show on Animal Planet right now about scavengers... they just finished up a segment about the vultures of South Africa (Cape Griffon, Lappet-Faced Vulture and the like). One of the things they felt it necessary to point out was that the old-world vultures and new-world vultures are externally similar, but vastly different. Old-world vultures rely entirely on eyesight to locate and evaluate food: they either don't have or have an extremely limited sense of smell.
I don't know why the distinction is all that hard to accept. The wikipedia entries on new- and old-world vultures explains that they evolved from separate families to perform the same function. This isn't surprising: both have powerful feet, broad wings, and a lack of feathers on the head to aid in cleaning.
The difference comes in lineage: old-world vultures are members of the accipitridae family which, as you might guess if you know of hawk classifications, are the birds of prey. This is so plain upon inspection that I don't know how you could make the mistake of believing that new-world vultures are at all similar. Old-world vultures, best exemplified by good shots of the Cape Griffon, have their eyes set forward in their heads and a very pronounced hawk-like beak, hooked at the end.
The vultures you find in America (Turkey and American Black vultures) have their eyes set on the sides of their head and have long, slender faces and appropriately sized and shaped beaks. They're much more slender in that regard, though their heads are much thicker on their necks (at least, upon cursory inspection they are).
Rants like this are probably the key to understanding my way of thinking.
[edit from 2019: unsurprisingly, I ended up volunteering at the national zoo for five years, during which time I got to explain the concept of convergent evolution to literally tens of guests]
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