gho64,
generally, you want to use curves, levels, contrast/brightness, masks or selections to isolate just the blown out areas, and possibly channels. but, as delic points out, if the area is 'blown out', meaning basically a pure white of 255, especially across a large area, then the only handling is to re-shoot the picture or do some detailed hand editing. curves and those tools can only handle things correctly if there are differences in the whites or hues. if it's all monocolor or all white or all black, then curves cant make any distinctions.
curves is a way of handling different values of color and black and white in ranges of those values. the scale is horizontally you are dealing with the hues or shades, while vertically you are dealing with intensities of those shades. the diagonal dashed line is the medium for all that and that is what you move. if you click on the middle of the diagonal and pull it straight down, you are moving that hue/shade lower in intensity, but since the line is connected always, the rest of the line also dips and thus also lowers all those other values represented by the line. the beauty of this is that you can set more than one point and fix the line so it doesnt move all over. thus, you can set a point on the middle and then move the top quarter of the line and the line will lower all those top values but not the lower values because of the fixed point you set in the middle.
so, in your case, if you have areas that are truly 'blown out' you can move the line down and lower the values of the blown out area, but the whole area is going to move down because the values are all the same, 255. so you could drop all the 255's down to 240, but you cant separate one 255 from another; not with curves, anyways.
hope that made sense
craig