The trademark was filed for on September 29 and lists Valve as its owner. It's a standard trademark filing, offering little more than the name, owner, and classification, which, as you'd expect, describes it as "video game software."
Countless Half-Life rumors have surfaced over the past half decade, with little or no official word to be had from Valve, which had actually set out to reduce the time in between Half-Life releases by turning to an episodic format following Half-Life 2. Last year, there was talk of an open-world Half-Life game. More recently, internal documents from Valve suggested that Half-Life 3 and Left 4 Dead 3 might both be in the works.
A trademark filing alone guarantees nothing, but after such a long drought, this does at least give Half-Life fans reason to feel optimistic.