Despite various dictionaries having wimpish definitions of unsustainable (e.g., from dictionary.com), Michael Pollan (in The Omnivore's Dilemma) has a great definition: "Sooner or later it must collapse."
Depletion or damage of a resource sounds so reversible; collapse sounds a little more final.
I heard that cyclists (who cycle rather than drive) do not save fossil fuel energy overall. They are healthier, and live longer, and so cook more, heat their houses longer, use appliances for more years ... overall, it is a wash.
Also, some cyclists (naming no names), go out and buy fancy recumbent bikes, bling-bling GPS devices and the like. They then drive their bike to suburban bike paths, cycle up and down, and drive home (via the local instantiation of the probably unsustainable Starbucks corporation).
At least if I call myself a hypocrite I have the cover of the Epimenides Paradox (similar, I believe, to Bertrand Russell's set-of-all-sets paradox) to hide behind, by blinding the world with logic.