Cyan has added Myst V: End of Ages to Steam. The title is $9.99, but is presently on sale for $8.99. It's Windows-only, so all you Mac users looking for a way to play End of Ages on your Intel-based Lion machines (all three of us...) are unfortunately out of luck.
Cyan also announced that the game will be coming to Good Old Games as well, though at this time it's not yet available there.
[ Myst V: End of Ages on Steam
[ Press Release