Post-copular DPs do not need Case because they are not arguments of copular be. Such DPs, as Atamiri says, are (one-place) 'predicates' or constituents of 'predicates' (see Bowers' alternative analysis infra), their only argument is, indeed, theta-marked internally to the DP, at some stage occupies the Spec DP position, and, eventually, raises into Spec T/Infl and becomes the subject of the copular sentence.
Note, however, that this applies only to cases of copular be, not to those in which be expresses identity (i.e., 'A is B' : 'A = B') as in That tall woman is John's wife / John's wife is that tall woman, where be is a two-place verb with two 'reversible' arguments, one in complement position and the other - under the VP-Internal Subject Hypothesis - in Spec VP (= Spec be). Whichever of the two DPs functions as complement of 'identificational' be does need Case, of course, but that causes no problem, because dyadic be is transitive-like and assigns (accusative?) Case to its internal argument.
Copular be, on the contrary, selects just a non-verbal one-place predicate as its complement, i.e., minimally a 'predicative' DP in the cases you are interested in. The hedge 'minimally' is not gratuitous, though, because there is fairly compelling syntactic evidence that the complements of 'copular be' are not directly 'predicative' DPs, but Predication Phrases headed by a 'functional' Pred head that selects a DP as its complement (i.e., structures like [__ Pred [DP]]) and assigns Case to it. (See John Bowers' 'The Syntax of Predication', Linguistic Inquiry 24/4 (1993), pp. 591-656, or his more recent summary 'Predication', in The Handbook of Contemporary Syntactic Theory, Blackwell 2001, pp. 298-333 for classic statements of this alternative view).
