Yeah, impulse 81 is correct. It turns the SMG into a set of cubes to test reflectivity.
As for the cubemaps, I think it's a DirectX 9 feature. If your GFX card can't handle DX9 (i.e. It wasn't built for DX9) then HL2 won't run at DX9 mode. I think that's why Snpbond hasn't noticed a difference.
A couple of days ago I force loaded HL2 to run with DX9 and the models were indeed black since I hadn't built the cubemaps. Normally, under DX8 mode (my GFX card's natural DX version), the models look fine and the cubemaps do little difference.