In October 2011 the premier support for Sun Cluster 3.1 ended. See details in Oracle Lifetime Support Policy for Oracle and Sun System Software document. There no 'Extended Support' and the 'Sustaining Support Ends' is indefinite.

But for indefinite 'Sustaining Support' I like to point out from the mentioned document (version Sept. 2011) on page 5:

Sustaining Support does NOT include:
* New program updates, fixes, security alerts, general maintenance releases, selected functionality releases and documentation updates or upgrade tools
* Certification with most new third-party products/versions and most new Oracle products
* 24 hour commitment and response guidelines for Severity 1 service requests
*Previously released fixes or updates that Oracle no longer supports

This means Solaris 10 9/10 update9 is the last qualified release for Sun Cluster 3.1. So, Sun Cluster 3.1 is not qualified on Solaris 10 8/11 Update10. Furthermore there is an issue around with SVM patch 145899-06 or higher. This SVM patch is part of Solaris 10 8/11 Update10. The 145899-06 is the first released patch of this number, therefore the support for Sun Cluster 3.1 ends with the previous SVM patches 144622-01 and 139967-02. For details about the known problem with SVM patch 145899-06 please refer to doc 1378828.1.

Further this means you should freeze (no patching, no upgrade) your Sun Cluster 3.1 configuration not later than Solaris 10 9/10 update9. Or even better plan an upgrade to Solaris Cluster 3.3 now to get back to full support.