Good tips. You can also ctrl-click on a channel to make a selection on the white in that channel (and fuzzy-selecting the grey).

Depending on how you're using your scanned line art, you may not even need to delete the white pixels. Set the line art layer to the multiply blendmode. The white will vanish, leaving nice, clean lines with no aliasing problems. It won't always be the solution you need, but it works a lot of the time, and it's non-destructive, so you can still apply a curves adjustment to modify the amount of detail in your lines.

I like the Quick Mask for making gradient selections. It's handy for adding haze to an environmental image or for simulating a pool of light.