I'm not a supporter of sporting boycotts in most cases. The exceptions can be distilled down to two types. One, where discrimination and exclusion is inherently carried out by the host sporting body (eg. apartheid-era South Africa). And two, where the host nation is in such a repressed, run-down state that playing international sport in those surrounds would be grossly immoral (eg. Zimbabwe, North Korea).
Boycotts of the Olympic Games are almost as old as the modern Olympics themselves. Think of Irish athletes refusing to compete in the first London Games and you realise that 2008 is the centennial year of the Olympic boycott.