Ridley's "Evolution" textbook says (3rd edition, p.4):
Developmental change within the life of an organism does not count as evolution in the strict sense and the definition referred to evolution as "a change between generations" to exclude developmental changes.
Of course, generations of a multi-cellular organism is one thing, and generations of its cells are another. Developmental changes can indeed be a form of evolution from the perspective of somatic cell generations. Now it looks as though the evolution of cancers is turning into another well-documented example of Darwinian evolution during development which involves somatic cell lines. Some quote illustrate the cancer-related discoveries:
Over the last 2 years, there have been an unprecedented number of publications focused on cancer evolutionary processes in solid and haematological cancers, a trend that is set to continue over the next decade. [...] It is increasingly clear that many advanced tumors follow a branched, Darwinian evolutionary trajectory. This has been demonstrated in childhood ALL , pancreatic cancer [2, 3], colorectal cancer , clear cell renal carcinoma [5, 6], breast cancer [7, 8] and prostate cancer  among others. [source]
Cancer development within an individual is also an evolutionary process, which in many respects mirrors species evolution. Species evolve by mutation and selection acting on individuals in a population; tumors evolve by mutation and selection acting on cells in a tissue. The processes of mutation and selection are integral to the evolution of cancer at every step of multistage carcinogenesis, from tumor genesis to metastasis. [source]
Iconic examples of evolution (birds evolving from dinosaurs, hominids evolving an upright posture, or a lineage of lobe-finned fish evolving four legs and moving onto land) might seem unrelated to the growth of a cancerous tumor, but the process underlying them both — natural selection — is identical. We typically think of natural selection acting among individuals, favoring those carrying advantageous traits and making those traits more common in the next generation. However, the key elements of this process — variation, inheritance, and selective advantage — characterize not just populations of organisms in a particular environment, but also populations of cells within our own bodies. The cells lining your intestines, for example, are not genetically uniform; there is variation among them. [source]
A wikipedia page offers a summary of the topic.
The topic is receiving interest partly since evolutionary theory is involved in the treatment of cancer.
Somatic evolution becoming more orthodox will help with acceptance of cultural evolution. For one thing, once the textbooks get rewritten, people will no longer be able to point at them, saying that changes to individuals over their own lifespan do not count as being 'evolution' - by the definition of the term.