The last I heard OpenDNS was using it for some data processing and Skylight was using it - see "Is Rust Ready Yet?" in http://cmr.github.io/blog/2014/01/12/the-state-of-rust-0-dot.... This is 9 months old now, so maybe there are more projects now.

That said I think the language is still too much in flux to really use for production systems. It seems to still be undergoing syntax changes, standard library changes, etc that might make code that works today not compile tomorrow.