Upgrading SQL Server 2008 failover cluster node to SQL Server 2012 failover cluster node
It's really not THAT different to Service Pack patching whereby you'll need to do passive nodes first, then give cluster resource group ownership to one of the upgraded nodes, in order to finally upgrade the active node.

Map out your upgrade sequence first and test it if you have the luxury of a dev environment. 

In place upgrades are supported and the information is already out there from Microsoft.

See here for example.

You'll probably want to perform what's called a rolling update, which can be best explained by the following paragraph from the article:


  To upgrade a SQL Server failover cluster to SQL Server 2012, you must run setup with upgrade action on each failover cluster node, one at a time, starting with the passive nodes. As you upgrade each node, it is left out of the possible owners of the failover cluster. If there is an unexpected failover, the upgraded nodes do not participate in the failover until cluster resource group ownership is moved to an upgraded node by SQL Server Setup.


Honestly, the biggest headache I've had with this is with application compatibility and agreeing downtime. The process was simple for me and my nodes...