Oh, absolutely. Enabling other CAs to use the ACME protocol* for certificate provisioning was an explicit goal during its development. As such, it's incredibly disappointing that Startcom decided to build their own incompatible solution here.
*: Yes, that's really what it's called. Stands for "Automatic Certificate Management Environment".
The only two servers I'm aware of is the LE server and the LE staging server which both run the same software.
It would be really cool to get another CA giving out certs using the protocol.