What are the hazards of upgrading a SQL Server service pack?
Well one thing you may want to consider doing is validating your application(s) still work on a test instance that contains the same database and upgrading it to the newer service pack. While rare and usually changes are backward compatible, there could be behavior changes to things like the optimizer that would be impossible to guess exactly how they might affect your application without testing. The risk is certainly less with a service pack than a version upgrade, for example, but installing a service pack on a cluster does not exactly have a trivial "undo" button - so in this scenario the backups won't really help you directly (they are still an absolutely sound idea, of course).