The popular virtual world Second Life is now publicly testing a Mono viewer. When in a Mono region, this viewer allows LSL scripts to be compiled against Mono. In theory, this will provide reduced lag and improved stability for Second Life users. According to Linden Labs, early results are promising:
We've run some benchmarks to compare the performance of the LSL2 virtual machine and the Mono VM. When we run the tests side by side we found that Mono is up to 220x faster than LSL2. These benchmarks were math intensive scripts usually used to evaluate performance. Once we get the beta program going we hope to get input from residents on Mono performance with more typical SL scripts.
Script performance is an essential consideration for Second Life. Unlike most online worlds, Second Life relies heavily on user-generated scripts. Even the simplest objects may have some scripting attached to them and a given server may be running hundreds of them simultaneously.
Since the LSL byte code isn't directly translatable into the IL byte code used by Mono, only scripts that still have the original LSL source code can be ported. Also, the Mono VM only accepts raw source so no "pre-compiled" .NET code can be inserted. Presumably this could change if Linden Labs follows their current plan of eventually opening it up to other .NET languages.
It was first announced that Linden Labs was considering Mono in August 2005.