I'm sure it's in a EULA somewhere, but it basically states that Microsoft does not allow beta software to be run in a production environment. The reason is Microsoft doesn't want companies making money off of free software. But I guess it really depends on the term "production". As if they're still testing the OS, even if they are doing production work, I'd still consider it a "test" environment.
But seriously, no one from M$ is going to come to your business and try to prove that your "testing" of the beta OS is being used for profit. It's just there for legal reasons and is not going to cause anyone a problem unless they convert their entire organization over to the beta/RC. You'd have to be pretty stupid to do that anyway as...