It's looking more and more likely that we won't see it until after E3.
With the announced source code leak a few months ago, and other problems associated with the development of this much anticipated shooter, reports are circulating that Half-Life 2 may not see retail shelves until the Summer of 2004.
This would give Valve a few more months to market and push this title to gamers, as if it needed anymore help. That and an E3 2004 appearance might be enough for some fanboys to crown this game title of the year without it even hitting retail.