Source: PlanetHalfLife
Related: They Hunger

The creator of very popular zombie mod trilogy They Hunger, Neil Manke, reports that it's still unsure if a Source remake will be made:

We have not received a reply to our inquiry to Valve concerning remaking "They Hunger" using the HL2/Source engine. So, at least for the time being, it looks as if we will not be doing a HL2/Source project.

I for one do hope that the They Hunger trilogy will be remade for the Source Engine. More on this as it develops.