Just looked this up and found the answer...you can reference the count of inline form validation errors and use that in a condition display. Now you can just use the select box for (No) Inline Validation Errors Displayed, but what if you want that and something else (not null, 1=2, etc)?
Just add "and wwv_flow.g_inline_validation_error_cnt (<,>,=) x" to a plsql condition/function. Now you can create more complex conditions based on form/page validations!