Mosh stands for "mobile shell", it is an extremely convenient alternative to ssh (although it uses ssh for authentication and stablishing the initial secure channel), specially for high-latency connections, as it is based on UDP and have some "premature echoing" that makes the connection feels very responsive, and it recovers instantly from network problems.
For more information: http://mosh.mit.edu/
I guess adding support to cssh should be trivial (there are workarounds to make it work by just replacing the ssh command in the config file even, http://www.dopefish.de/archives/1138, but it would be really nice to have out-of-the-box support for it.
Thanks!