Will be released in early 2010.
Croteam and Devolver Digital have announced that Serious Sam HD: The Second Encounter will be released in early 2010 for PC and home console - presumably Xbox 360 given that's the console on which First Encounter will eventually be released.
Second Encounter's HD remake will feature all the insane action of the original, but with modernised visuals. The remake will also feature an all new Co-op Tournament mode, allowing teams to compete with one another.
The original was released back in March 2001.