As it is with every SQL Server release, 2012 also has a white paper which covers in details the process of upgrade to SQL Server 2012. I’m sure that by now a lot of people have already tried it in their labs but it’s good to have a document that describes whole scenario with risks and possible caveats.
The document is available here. I strongly recommend reading it not only because of technical stuff (which is obviously useful in one place), but for project management-related information it contains. It also features ready-to-use checklists you migt want to incorporate into you upgrade projects.