I posed this question to Brain Boost last week, and I can't stop thinking about the (possible) answer. It's not really current news, but it's so interesting I'm posting it anyway:
"A purring cat is not necessarily a happy one; many species-including cheetahs and some lions-also purr when wounded or anxious. Some researchers speculate that this lovely rumble may serve a function: to heal fractures and strengthen bones. In an as yet unpublished study from the Fauna Communications Research Institute in Hillsborough, N.C., investigators determined that the frequency at which many cats purr, between 27 and 44 hertz for house cats, matches the frequency that seems to help human bones strengthen and grow. If correct, the theory may explain why cats heal so quickly after injury." -Scientific American; June2001, Vol. 284 Issue 6, p32.
I can't find the study (I think it's still unpublished), but you can read more about it in this Scientific American article (Hampshire only at that URL).