What the hell could it possibly be?  And just imagine the release date.  Say Ep 3 comes out in Christmas 2009; Valve will have taken 5 years to make 15 hours of gameplay on an old engine to boot (incremental upgrades to Source or not, its old).  I'm not really complaining about the game quality, since I love HL2 to death, but the waiting is interminable.  Predicted release date for Half-Life 3: 2016

Well Half Life 2 Episodes are essentially Half Life 3....any future game apparently is going to be titled something else other than Half Life 3 i think. But one thing for sure is to expect delays from Valve regarding a release date, if they give a set date add another month or two onto it as they are notorious for delaying games. That being said i do agree the Source engine is old, its not bad and competes with the newer ones rather well, but it really needs a whole revamp. I have been wondering if the reason for Half Life 2: Episode 3 having so little news revealed about it is because Valve may be working on either A.) Overhauling the current engine, making it work amazingly better and improve it visually or B.) They have started work on a new engine and perhaps want to release it alongside the final game in a star studded series.