debian/additions/my.cnf file turns on log-bin by default:
Unless it was done on purpose, it's not a great idea because
a) it obviously affects performance for those who doesn't need the binlog;
b) it causes unpleasant side-effects, due to statement logging being unsafe for various configurations and statements (warnings in the error log, errors on using InnoDB + READ-COMMITTED);
c) it's different from MySQL setup.
If it's possible to do it carefully now (without overriding custom cnf files on upgrade), it would be good to fix it in 5.5. In 10.0 it should be safe enough to fix, since it's only alpha.