Carbon nanostructures have been shown to embed themselves as fibres into tissues, causing irritation and possibly cancer. So even if buckyballs are themselves harmless, likely carbon contaminants from the production process might not be. The balls also have a tendency to encase small molecules within them, and then delivering those encased substances directly into the human cell. So special care would have to be taken that the buckies are really "empty". That's all under the assumption the C60s are themselves non-toxic.

Some fullerene compound substances are toxic, but the C60 pure fullerene seems to be OK so far. Still, I wouldn't bet my life on it. If, for example, the C60 bucky would have a toxicity mechanism comparable to carbon nanotubes, that may be damage unlikely to show up during the limited lifetime of a rat - but it may well be much more relevant to humans who live 30 times as long!