I've diagnosed the heck out of this issue - the Import form is simply not posting to "admin.php?import=wordpress&step=1"
If the call to the function wp_import_upload_form is changed to be any other URL, the form will post to it, however, posting to Step1 fails. Not only does it fail, it simply does nothing- no function is instantiated, no PHP process is spun up (monitored via Top), just NOTHING.
I've put breaks on the admin.php in sections and returned the step in Import functions requirement checks, etc., and that all checks out. It's as if SOMETHING is taking the Post request and killing it - it's not even making it to request the URL - just spins the browser request - and the "dispatch" function is not hit again to determine what step you are on.
I've done the usual diligence - turned off all plugins, etc...no dice.
Anyone have an ideas?