I tried the "formatwithmbr" option as well. On ESXi 5.0, I needed this and it worked.
On 5.1, I used the option - install completed - but I still have the same problem reported here.
Even more interesting, if I enable UEFI boot - I do get the initial boot (with a later purple-screen, if I continue). This tells me that the formatwithmbr option is being ignored. More proof: if I boot from my device in this mode and press shift-O - the boot options clearly show a boot UUID.
Perhaps the formatwithmbr option has been removed in the ESXi 5.1 installer?
To test, I'm going to re-format with ESXi 5.0, which worked in the past. Then, I'll try an upgrade rather than a clean install.