"In a recent interview with StuffWeLike, Valve's Doug Lombardi confirmed that the wide world of Half-Life won't end after Half-Life 2: Episode 3. There's more on the way: plenty more.
Question: "Are there any current plans after Episode 3 to have a Half Life 3?"
Answer: "We haven't announced anything specific, but Half-Life won't end at Episode Three - hang on to your crowbars!"
"Half-Life 3 or Episode 4? That's fine with me, especially since the sequels haven't stopped Valve from putting out other incredible products with the help of Half-Life technology. If a tried and true franchise allows Valve to experiment with the mechanics you see in Portal or the artistic choices you see in Team Fortress 2, I say bring on the sell-out! I'll be waiting with my Mt. Dew in the ready."