Pubsubhubbub - Subscription was refused by your callback url (or it couldn't be reached)

Thanks for your quick and helpful response. Please keep in mind Alex that I'm on the steep part of the learning curve with much of this.

I've had mixed results exploring your suggestions, but ultimately figured out how to successfully subscribe to my Superfeedr hub:

* Have you tried pointing feeds to Google's hub (e. g. by subscribing to a feedburner feed). Do you get the same error?
No - I don't get the same error (http://dpreble:XXXXXXXX@superfeedr.com/hubbub vs. http://pubsubhubbub.appspot.com/). Continue reading.

* Have you added your username/pw to the superfeedr url? My superfeedr URL looks like this: http://alex_b:xxxxxxx@superfeedr.com/hubbub
Yes, I had this set correctly.

* Use HEAD as I've fixed some smaller problems with PuSH (that don't seem to be an issue here).
I ran into trouble using HEAD, but think I may have confused my problem with this other issue: "Can not delete feed_item content type" (http://drupal.org/node/654326).

* Try debugging the problem better with feeds_dbg() and tail -f (see feeds.module for a better idea of what you can do w/ feeds_dbg and where you can locate the log on your local file system).
When it finally sunk in that the response from Superfeedr.com containging the hub_challenge was not something I could troubleshoot using xdebug and that I should not only set the feeds_debug variable to TRUE (using Drush) but also sprinkle feeds_dbg('Infomative ' . $messages) liberally throughout the code, then I started getting somewhere. At first, I determined the problem was in the verifyRequest function of PuSHSubscriber.inc, but in the end, the real culprit was this line:

File: PuSHSubscriber.inc
Function: protected function loadSubscription()
Line: return call_user_func("{$this->subscription_class}::load", $this->domain, $this->subscriber_id);

It seems different versions of PHP require different syntax for call_user_func. When I change this line to the following, everything works great:

return call_user_func(array($this->subscription_class,'load'), $this->domain, $this->subscriber_id);

I'm using PHP 5.2.0. Funny thing is I had been getting an error message about this line when I tried deleting feeds ("Unable to call PuSHSubscription::load(superfeedr,x"), but I figured the error was due to the fact that the feeds were never being successfully created, and would go away when I solved the creation problem - turns out it WAS the problem.

Okay, now for the somewhat unsettling part. Even with the line that did not work with Superfeedr.com, the Google hub was returning a successsful result:

Superfeedr returned: Code 422
Google returned: Code 204

With the line edited:

Superfeedr returned: Code 204
Google returned: Code 204

The Superfeedr results make more sense to me - it's almost as if the Google hub takes whatever response you send and says "good enough for me". Any thoughts?