What are the hazards of upgrading a SQL Server service pack? Are there any specific pitfalls/standard considerations when upgrading an instance of MS SQL Server to a higher service pack? Is there anything special that is generally done besides making sure there are current backups of system and user databases?

I'm asking the question in general since I presume the answer is similar regardless of version or situation. However, in the case that I'm mistaken and the specifics do matter, the situation that prompts my question is an upgrade of SQL Server 2008 from SP2 to SP3, running on Windows Server 2008 with failover clustering.
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).