Apply SQLServer Service Pack to a Single Node Cluster
Nothing particular to worry about, but you will incur downtime as the SP runs its stuff in "script mode" during which users cannot connect.

It might go without saying, but I'd ensure you have a valid (i.e. tested) backup of the databases (including the system databases) on the affected system prior to doing any patching, whether on a single-node cluster, a multi-node cluster, or even a stand-alone instance.  You can never have too many backups.