How can I debug a Views 2 block argument set via "Provide default argument" and "PHP Code"?
After more testing it appears that the watchdog function does work for debugging the embedded PHP code in the Argument Validator and Argument Default Value. I believe my validation was too strict previously and was preventing any output. For example embedding the line below in the "PHP validate code: " area gives all the argument components.

watchdog('block-args-php', var_export(arg(), TRUE));