hmm, I don't see any harsh, sharp edges there. Only a slightly darker patch in the lower left (leading up diagonally to the upper left corner) - I guess this is part of the image though.
Clearly, it's not a res problem, and there's no real banding there.
I've and idea
Open the saved for web image again in photoshop (with a decent res, enough to zoom in a little if needed) If that appers OK, select a soft edged clone-stamp brush, and try cloning the gradient. Does it clone it exactly? I don't know if I'm deviating too far from the problem here, but trying to narrow it down in my mind to what's causing it and why.
I'm on a macbook (LCD) and that image appears to have a smooth gradient to me. Perhaps it is something to do with calibration, but you seem to really know what you're doing here, which makes it all the more elusive
hmm.....
Can you take a snap shot of the problem area (zoomed out a little) and put it beside the saved for web version? If we also see a difference then, it might be easier to narrow it down.