Been having problems with a GF + PayPal + MailChimp form. Thought it might be related to some IPN trouble we had last month (Bad Behavior was blocking the IPN calls), so I turned on GF's debugging log.
The log shows the IPN is coming in okay, but is missing the custom field: "IPN request does not have a custom field, so it was not created by Gravity Forms. Aborting."
Suggestions welcome (and will be rewarded with chocolate :-) )