To answer my own question: It works well with ESXi 5.1.
Just add the intermediate certificate to /etc/vmware/ssl/rui.crt after the server cert.
I tested the followong clients/browsers:
- vSphere Client on Windows 7
- IE8 and IE9 on Windows
- Safari and Chrome on iPad/iOS 6.0
- Internal browser, Chrome and Firefox on Android 4.0
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