If you own Illustrator CS2, you have an extra tool that can easily be used to create interesting shape effects. (This assumes you know at least the basics of using Illustrator, of course.)
illustrator to photoshop
Create a basic rectangle in Illustrator filled with black, and then add some extra paths, stroking them using some of the wonderful brush libraries, such as Artistic-Chalk Charcoal Pencil. Select everything (press Command-A [PC: Control-A] to Select All) and choose Edit>Copy, or press Command-C (PC: Control-C).
In your Photoshop document, press-and-hold the Option key (PC: Alt key) and double-click on the Background layer to unlock and rename it. Then, hold down the Command key (PC: Control key) and click on the Create a New Layer icon to add a new layer below the renamed Background layer. Press D to set your Foreground and Background colors to their defaults, and fill that layer with white by pressing Command-Delete (PC: Control-Backspace).
Press Command-V (PC: Control-V) to Paste the copied Illustrator artwork. In this example, we'll use the Paste As Pixels option to create a layer based on the copied Illustrator artwork.
If necessary, use the transformation handles to resize the pasted graphic (press-and-hold the Shift key and click on the corner handles to resize proportionally), and then press Return (PC: Enter) to finalize the size.
Drag the shape layer in between the white layer (Layer 1) and the photo layer (Layer 0). Click on the top layer (the photo layer) and then from the Layer menu choose Create Clipping Mask or press Command-Option-G (PC: Control-Alt-G). The photo layer will only be visible within the pixels of the shape layer immediately below it.
In order to edit the shape of your photo, you have to either add more pixels to the copied Illustrator layer where you want the photo to show, or erase pixels in areas where you don't want to see the image. To give yourself a little more flexibility, consider turning the shape into a layer mask.
If you've already pasted the shape from Illustrator and pressed Return to finalize the size, here's what you do to create a layer mask: press-and-hold the Command key (PC: Control key) and click on the thumbnail of the shape layer to make a selection in that shape. Click on the photo layer to activate it, and then click on the Add Layer Mask icon at the bottom of the Layers palette.
If you want to go directly from Illustrator to creating a layer mask, well, you can't. You'll have to paste the graphic, hit Return, and then follow the steps above to make a layer mask.
Creating a layer mask offers one big advantage over the clipping mask: you can apply filters to or paint on the layer mask to change the mask.