Based on recently proposed antiferromagnetic spin fluctuation exchange
models for d(x2-y2) superconductors, we show that coupling to harmoni
c phonons cannot account for the observed isotope effect in the cuprat
e high-T-c materials, whereas coupling to strong anharmonic multiple-w
ell lattice tunneling modes can. Our results thus point towards a stro
ngly enhanced effective electron-phonon coupling and a possible breakd
own of Migdal-Eliashberg theory in the cuprates.