If you work in image ready or photoshop, you can do that from the layers palette. You can show it pressing F7 or from the windows> layers menu.
Select the layer you text is on, look at the top of the palette and search an "opacity" textbox. Lower the setting and you're done.
If the text has to overlap a transparent background, you may be better off using a layer mask, though.
http://www.lunacore.com/photoshop/tutorials/tut003.htm