Is there a tool for reformatting C# code?

You could also try NArrange to reformat your code. The formatting options it supports are still pretty limited, but it can process an entire directory and is a command-line tool. Also, NArrange runs under Mono.

You could give Artistic Style a try. It requires Perl to be installed though.

It's got a decent list of formatting options, and supports C and Java as well.

This isn't command-line, Mono or Linux, but it's something: I've been using ReSharper (made by JetBrains) and it's rather good. It's a Visual Studio plugin, so I'm guessing it's not your cup of tea.