Found it!
Did a search and someone else had changed their fsockopen call to:
$fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);
I tried that and it worked.
Once again, I'm confused as to how it is (was?) working with Joomla 1.0. These don't appear to be Joomla 1.0/1.5 incompatibility issues.