Suddenly, my HP Laserjet 4MP+ started printing light grey everything. Toner was fine, printer settings all fine. Then I remembered I'd installed 10.2 since using the printer last.
The problem was that because Jaguar had recognised the printer OK, I hadn't installed the HP 4,5,6 drivers for Mac OS X from the HP site. Once I'd done that, proper blacks were immediately available again.
Not a sophisticated 'tip', but one that might spare others some anguish!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021017063301349