Provided you have an existing R2 cluster...
Create a domain service account and give it "Log on as a service" on each node.
Open port 5151 in the Firewall of each node for the SDE service.
Install ArcSDE on both nodes. ( Do not Run post- install on second node!)
Run the Post Install wizard and select complete.
Use the cluster sql name\instance to create the DB and repository.
Complete the authorization wizard using the sql cluster name.
When creating the service enter the domain account info from earlier . You will also use the DB name created in the earlier wizard. Use the SQL clustername\Instance for "SQL Server Instance name" and SQl Cluster name for "Server name".
After post install is complete make sure the service starts, and that your domain account is starting it. Then stop the service and change startup to "Manual".
Run post-install on second node and install the service only using settings in previous step. (Do not start the service when prompted) Change service to "Manual"
Open Failover cluster manager and right-click the SQL Server () and "Add Resource" followed by "Generic Service"
Select the ArcSDE service and finish the wizard.
Right click the newly created resource and select properties .
Delete contents of the startup parameters box
Add SQL Server Agent as dependency.
Optional if using SQL for other purposes besides ArcSDE
Uncheck both boxes under "Policies tab" so the SDE service will not cause a failover.
Right click the SDE service resource and bring online. If it is successful test failover.