Good question Chris.
I’ve found that the more hoops you make your users jump through, the greater the chance they’ll find a workaround. I haven’t seen any clients take the sandboxed email client approach but it’s worthy of consideration. I’d just recommend making sure you think things through first and get management on board before make such a change. Don’t want to be the scapegoat!
I still believe that many mobile risks can be reduced by following some common sense guidelines like I talk about here:
Top 10 reasons we have our heads in the sand over mobile security
Securing the new desktop: enterprise mobile devices
It’s time we shift our thinking about endpoint protection
The cloud is another issue altogether since you’ve now lost most control. Here’s a related piece I just wrote about Hosted Exchange Server security considerations.
One thing’s for sure, I don’t envy IT managers these days!