While Apple does not have an official support policy with expiration dates (I find it rather weird that nobody is bothered by this), the unofficial rule seems to be that version n stops being supported (as in "security updates quietly stop flowing in") shortly after version n+2 is released. 10.7 is not even pre-announced yet.
I find OS X a more realistic choice for extending the life of Apple's PPC hardware: to keep Linux running on such hardware you need to do continuous beta (and sometimes alpha) testing as my experience with Fedora PPC shows. Needless to say, hardware support is far better in OS X too.
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