The popular virtual world Second Life relies heavily on user-created scripts. These scripts are written in a proprietary language called LSL and run on their servers. In order to improve performance they are deploying the Mono runtime in Second Life's 1.24 servers.
While Second Life is planning on supporting traditional .NET languages, they are not throwing away their current offering. Instead, LSL code is compiled to IL as if it were any other .NET compatible language.
Due to timing differences and minor differences between native and Mono-LSL, scripts will not be automatically upgraded. Second Life is expecting their large user base to test their own scripts and mark them if they are satisfied they work correctly under Mono. Those that are not upgraded will continue to run under the native LSL interpreter. Upgrading is as simple as editing the script and checking the "Mono" checkbox.