SAP own the free DBMS MaxDB - so aren't exactly dependent, although Sybase have a much more diverse product range. Interesting developments in the enterprise space.
Does SAP run on MySQL? I never used SAP but it seems 'enterprisey' enough to demand some pretty special DB.
Basically 90% of their customers are on Oracle, DB2, and MS SQL. Apparently they tried something with MySQL (http://www.sap.com/press.epx?pressid=2208) but never went anywhere.