Strange, I just copied and compiled that code without any problems on RH9 and Mono.
Code:
$ mcs Hello.cs
Compilation succeeded
$ mono Hello.exe
Hello World
$
Perhaps running mcs with the "-v" option will shed some light on this? Also, which version are you running (mcs --version)? I am currently using:
Code:
$ mcs --version
Mono C# compiler version 0.29.99.42746