This is a matter of much debate and disagreement, as old as copyleft licenses.
It's "strong" or "viral" copyleft vs. "weak" or "soft" copyleft. Traditionally,
the FSF takes teh side of strong copyleft with the GFDL, and the CC crowd tends
more towards the weak variant, implying that the share-alike requirement does
not apply to "aggregate" works, only "true" derivatives. To me, that makes more
sense in practice, even though it may be less desierable in principle. The
distinction is tricky, however.