PayPal Standard Recurring payments notifications not sent

Attached patch fixes the following error:
[Fri Jul 20 02:57:55 2012] [error] [client 66.211.170.66] PHP Fatal error: Class 'CRM_Contribute_BAO_ContributionPage' not found in /var/www/mysite/public/sites/all/modules/civicrm/CRM/Core/Payment/PayPalIPN.php on line 167

This seems to be the cause of the following message from PayPal:
Subject: PayPal Instant Payment Notification Warning
Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:

http://mysite.org/sites/all/modules/civicrm/extern/ipn.php?reset=1&contactID=37896&contributionID=14&module=contribute&membershipID=14&contributionRecurID=7&contributionPageID=1

[lots more - one for each recurring contribution setup in last few days]
If you do not recognize this URL, you may be using a service provider that is using IPN on your behalf. Please contact your service provider with the above information. If this problem continues, IPNs may be disabled for your account.

Thank you for your prompt attention to this issue.

Thanks,

PayPal

While I was at it I also added require_once statements for all objects in the file.