I know what you're thinking, "Why would you ever want to overprint black in Photoshop? Isn't that an issue for page layout or illustration software?" Well, yes, but lots of designers — even the developers themselves — are blurring the lines between what you're supposed to do in which program.
So, just accept that people are doing single-page layouts in Photoshop. They create a big blank canvas, plonk a picture upper left, type out a text block lower right, put some interesting background behind the whole thing, it's easy to do with layers. Or they're creating smaller works, mini-layouts — layered combinations of vector shapes, live text and raster images — that they'll place into an area of a larger page in InDesign, Illustrator, or QuarkXPress 6.5.
When designers use Photoshop in this way, they usually 1) Keep the final file layered — they don't flatten it; 2) Change the default rich black that Photoshop uses for black text to 100% K black, just like a layout program; and 3) Save the final file as a Photoshop PSD. If they're going to place the layered image into another program, they often save it as a Photoshop PDF instead, to prevent the type from rasterizing when output. (I wrote about this last bit way back in DesignGeek 24: http://www.senecadesign.com/designgeek/dgarchives/designgeek24.php#pdfsrule.)
Okay, so the issue is, most layout and illustration programs automatically overprint pure black text. If they didn't, the type would knock out of the background (think colored sidebars), creating a nightmare for press registration at smallish type sizes. If the text overprints then it's not a worry — there will always be ink behind the text, no matter how the paper slips on the press when it gets hit with the black plate.
But Photoshop always knocks out "only K" black text (or vectors, or raster pixels) from artwork in underlying layers when you print separations for 4/C presses. There's no Overprint Black setting in Preferences, because it's not supposed to be that type of program.
All that preamble for a very short solution: In Photoshop, change the blending mode of any pure black layer in the file to Multiply from the pop-up menu in the Layers palette. It has the same effect as setting that layer to Overprint, and since it's 100% black, it doesn't look much different. Try it — use the Channels palette in Photoshop, or the Separations Preview function in InDesign or Acrobat, to see for yourself.
One caveat: Page layout and illustration programs ignore Photoshop layer blending modes (that is, the modes don't affect other page elements behind the image) when you place or import them. So while "multiplied" black layers will overprint other layers within the image file, any black areas that don't encounter layer pixels underneath will knock out of any non-Photoshop artwork behind the image itself. If that's an issue, it's probabably better to stick with Photoshop's default rich black.