For some reason, some of portals textures show up as pink and black (with a slight overlay of the texture behind it) in hammer, but work fine when running the game or when viewed in the texture browser. It's driving me crazy. Anyone know whats going on?

Does this happen with just the reflective textures? (metal, glass etc.) In that case, Hammer is missing the default cube map texture that is normally used to mimic reflection. If it doesn't happen in-game, I personally wouldn't worry about it. Otherwise, you might try repairing the game GCFs. Try using this fix on both Portal and the Source SDK.

No, it actually doesn't happen on glass or metal. It happens with the concrete walls from portal and all the concrete floors except the light, non-checkered floor.

Have you tried the GCF fix yet? It might well be a missing resource problem, regardless of whether it's the cube map or not.

If that doesn't work, post a screen shot of the effect. Perhaps someone recognises the problem then.