Actually you're all right. It is in fact a layer mask AND a "Bump Map." It's just that Photoshop doesn't really have "Bump Maps."

What it does have is layer effects. You access them by double-clicking a layer in the layer palette...careful, in version 7, if you double-click the layer name, it just lets you rename the layer.

Anyway, once the (huge) layer blending options dialog comes up and almost totally obscures your image, there's an effect called "Bevel and Emboss" that features a couple of "sub-effects," one being "texture." The texture effect will apply a pattern of your choice as a 'bump map' of sorts. The effects dialog is a bit on the complex side, which it sort of needs to be, but if you have any questions after you've played with it a bit, feel free to email me.

Anyone stumping me with a Photoshop question gets a free pizza.

Oh, the good news is that in Photoshop, the default patterns include the one that it appears the artist used in that image, Satin...reversed. The bad news is that effect is not an easy one to achieve...it appears that he used a layer mask comprised of both an alpha of the dresses, a gradient, and a blur here and there. My 5-minute attempt to re-create (sort of), required the use of about five layers and six layer effects.