Back when SP2 became available for Project Server 2003, everyone learned a very valuable lesson in deploying service packs… ALWAYS deploy them to a test environment first! This not only allows you to (a) test the software update in a protected environment to see if the update itself does any damage to your data, but it also (b) allows you to test your update process.
Project Server 2010 SP1 has been out in the wild for over a month, and there have been no major issues published. If you log in with the proper account and carefully follow the installation instructions that are published, then it should be rather uneventful.
Always follow best practices such as taking the application offline and backing up the server(s) & databases before beginning, and remember that there are multiple service packs to be installed… for SharePoint, Project Server, and Project Professional.
