Wow, it's only taken me 3 years, but I finally have my Lexmark Z42 working in linux :)
It would always hang on me after printing a few inches...I figured it was a buffering problem somewhere, but it turns out that all I had to do was change the Output Mode from "Color" to "Raw CYMK" Of course! How could I have missed that? grrr...I love linux, but it still has a ways to go before it can take over the desktop world.
update (July 9) Well, maybe I spoke too soon...It's not as simple as just changing the output mode, sometimes this hangs as well...I can't quite figure out what's going on, sometimes an entire page will print, other times it will stop partway through (usually near the top, but the Gnome Print Test Page would reliably stop just before printing the colour circles at the bottom) So if anybody from the gimp-print or cups development team wants help debugging a Lexmark Z42, let me know :)