I had trouble getting a network printer running with cups. The logfile /var/log/cups/error_log said: E [01/Mar/2006:13:56:38 +0100] Unable to convert file 0 to printable format for job 5! I [01/Mar/2006:13:56:38 +0100] Hint: Do you have ESP Ghostscript installed? I [01/Mar/2006:13:56:38 +0100] Hint: Try setting the LogLevel to "debug". The package ghostscript-esp was installed, increasing verbosity didn't help. The fix can be found here: http://www.linuxprinting.org/error_log-faq.html#q_3_12 Translated into Gentoo lingo the above means: (re-)emerge the package ghostscript-esp and make sure that you have "cups" in your use flags. I also had to restart cupsd after installing ghostscript-esp.
Fixed in CVS. Added a new "Troubleshooting" chapter with this as the first section. Thanks to Guido for reporting.