They're immortal in the sense that they no longer listen to tumor suppression genes that trigger apoptosis (programmed cell death). Based on our current understanding of senescence, that doesn't really help with the main causes of aging.
The presence of telomerase prevents telomere shortening in cancer cells and gives the ability to divide an unlimited number of times in a laboratory cell culture plate.
But the aging process is not reversed, so a multicellular complex organism would eventually die.
