It's a good question. I don't think there are any widely used ones, but I'd bet there are a few running around for internal use.

I think that due to the nature of Linux in general the only ways to have a successful proprietary software package is by being a hardware vendor, owning a whole format that is widely used and needs to be licensed, or having pretty serious multiplatform support. Desktop environments don't really fall under these.

But I could be way the hell off the mark. I'm just a rambling drunk.