Last night we reversed the module load order in nsd.tcl so the verisign module (which subsequently loads the PayFlowPro libraries) is loaded *after* nsopenssl. Our server has been up for almost 24 hours, a record. I think that is at least a working solution. The real fix, of course, would be to get a copy of libpfpro.so dynamically linked against OpenSSL, but Verisign still has not returned our calls to level 2 technical support.
So for the record, it appears fixed. 😊
Thanks to everyone that responded here and on the AOLserver list. As I said over there, helpful people like you are what makes open source work.