SQL Server 2012 - Upgrade Whitepaper
Just a short note to mention Microsoft have released the Technical Reference Guide for upgrading to SQL Server 2012.
The paper is available for download here: http://tinyurl.com/84xm5b4
There's some interesting details on approaches to upgrade, including features such as high availability, full-text search, service broker and other components (SSIS, SSAS, SSRS). Additionally, there's a (fairly) recent initiative to organise and present TechNet content more easily, there's some useful content (with interesting presentation) at the link below:
Good luck planning your upgrades,