In a word, no. I have spent several days trying to find a clear answer the to this question in white papers, blogs and even MS Partner Support.
I think Microsoft will offer this service in the not so distant future because it just makes sense to. Using the Office 365 Feedback link (below) I posted the following question (which I may never receive a response to). If you want to use Exchange Online as a fail over to your On Premise Exchange, please surf to that feedback page and ask Microsoft to add this service:
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I am Partner ID: XXXXX and I have 3 customers each with Exchange 2010 who would love to use Exchange Online in a Hybrid solution as an Exchange cluster. The idea being that the users mail store would be on BOTH on-premise Exchange AND on Hosted Exchange. Basically, I would like the customers on-premise Exchange to fail over to the Hosted Exchange offered by 365.
I understand from support services that this not possible today (see Case# 123952XXX1). This seems very odd to me as you already have the infrastructure in place to make this happen and this would be more profitable than the pure hosted Exchange because much of the workload would normally be handled by the on-premise server. I have sold O365 since BPOS and know that you can not provide all services at the start… it takes time and many dev cycles to get enhancements made.
Sooooo, all of that leads me to this question: Does Microsoft have any plans to provide Exchange Online as a fail-over/high availability option?