Valve is not like other developers. They do not need Half Life anymore. They make more money in one year off games like Counterstrike and DOTA 2 then Half Life 2 made in it's entire lifetime. Not to mention how much money they make off Steam. At this point, they can never live up to the hype surrounding HL3 or whatever.

The only way it will happen is if technology grows enough to let them do something interesting. Valve has no interest in just putting out another Halflife game if it doesn't bring something new to the table.