That doesn't sound right, I've had Filevault on the entire time I've been playing Diablo 3... unless that's come in on the latest update cause I haven't played it in a couple weeks.
"This error is caused when FileVault options are enabled, or if a volume is formatted as case-sensitive. These options are not supported for installation."
"If FileVault is enabled, then Diablo III will not install."
I've had Diablo 3 running with File Vault, until a couple of patches later when it just refused to install the newest patch. Now, I can't even install it anymore. I'm unsure if it is a connection problem or related to File Vault. (Trying to install it on an external unencrypted Firewire drive failed too.)
"This error is caused when FileVault options are enabled, or if a volume is formatted as case-sensitive. These options are not supported for installation."
"If FileVault is enabled, then Diablo III will not install."