Ah, but security isn't windows-specific. It's essential if you want to use anybody's APIs that require SSL (and sometimes have broken certs). And it doesn't work in Mono today.
This could possibly be fixed by now, but last I tried, the C# sample code to interact with Amazon S3 didn't run under Mono.
I'm not familiar with the S3 .net api.. but linux doesn't have the notion of central certificate store like windows does. Therefore if you are relying on that existing, you are going to need to change your code.
It is certainly possible to use SSL with mono, many people do it.
This could possibly be fixed by now, but last I tried, the C# sample code to interact with Amazon S3 didn't run under Mono.