Myst V was announced earlier today, but there was no information about who was developing it or what kind of technology it would use. Someone just shouted the word "Myst" and then scampered into a hedge.
There's some meat on it now - Gamespot's got some goods. It's called End of Ages, and purports to draw the Myst series to a close. Cyan's on this one, not a team internal to Ubisoft like IV, and the screenshot provided doesn't look like it's using the engine from Four either. I wish I knew more.
(CW)TB