There's a very good TechNet article available that describes the steps that are required in order to fail over from a CCR cluster in one site, to an SCR target in another site. It also covers the procedure to fail back once the problem has been resolved
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Read the complete post at http://blogs.technet.com/ukmail/archive/2008/09/24/step-by-step-guide-on-failing-over-from-ccr-to-scr.aspx