nin, I'm just guessing but I expect that Steam will be required for online play and a source of patches for single-play. I'd be suprised if I purchased a copy of HL2 at retail that I would need steam to play the single player potion. I think this helps explain some of the retail packaging of HL2 into single-only vs. single & multi as the "full" version includes steam. I'm only speculating but I have to believe that Steam will fail miserably if it takes away freedoms users currently enjoy and I am confident that Valve understands this.
ZigZang