This is speaking from the standpoint of a prepress department for a printer. Sometimes RGB is okay, sometimes not. Most modern equipment will convert the RGB to CMYK at the output and things look fine. The big problem is type. When small black type is in a JPEG, TIFF, whatever, the conversion to CYMK results in process color type. Thus, unless it's printed on a four color press the type looks like crap. Now, if it's type in Illustrator, InDesign, Quark etc, overlaying a RGB image, then the software will result in black type over a process color image and all is cool. You would be surprised how many people design an entire poster in PhotoShop, type and all, and they can't see why the 6 point type at the bottom is fuzzy.