This is something I've been wondering for a while as I was using some Doom 3 models for testing my renderer.
Here's a screenshot.
Notice how down the middle, he has those seams. This is using my deferred shading engine and when I look at the normals in the Gbuffer, the normals are also full of seams there, while the diffuse output looks fairly OK.
The legs also have some nasty texture seam artifacts going on, and I can see these horrible seams when I render the specular gBuffer as well and see how the specular textures map on. The insides of the arms look especially bad.
I remember back in the day on my shitty graphics card, I'd have some seams down the middle of models and it seems to be a complaint around the web people have had. Forcing the normal maps to be uncompressed seems to have been the fix, and nowadays when I play Quake 4 and Doom 3 on higher than ultra settings by tweaking some cvars, I never see these seams.
My renderer is just loading the normal maps as they are in tga format in the .pak. I saw some dds textures in the paks as well and I assume Doom 3 actually uses those, but I don't have a dds texture loader just yet. (I'm also surprised at how much scratch work and useless stuff is inside the pak files that isn't really used by the game, but I'm also really greatful it's there)
Here's a forum post where they actually say to enable some sort of compression on normal maps.