I use apposition to refer to a phrase where two nouns or nounphrases are either the same or similar: Einstein, the physicist,
I see no difference between the cases except in terms of position:
Einstein, the physicist, said that ...... (nominative)
She interviewed Einstein, the physicist, (objective)
Commas are generally used, but are also omitted in examples like the following:
n : a grammatical relation between a word and a noun phrase that follows; "`Rudolph the red-nosed reindeer' is an example of apposition"
Source: WordNet ® 1.6, © 1997 Princeton University