<p>I was currently developing a desktop application in C# using mono and testing in SuSE and Windows. The concept doesn't matter much, but it is a "web lint" program that will scan a web site and return possible issues with security and/or cross browser compatibility.  However, I want to be able to offer binaries for multiple platforms.  Should I stay with Mono, or is there another platform that would give me a better availablility of platforms, such as on Macs, Windows, Linux, and others (possibly mobile platforms), and make it easier to port?  </p>

Java will run on Windows, Linux and Macs. Should be easy to transition from C# - use Apache HTTPClient for grabbing the web content you are scanning, and the scanning code should be more or less the same. However the downside is requiring the user to have the Java runtime installed.
Python is another option - you can build stand-alone executables for Windows, and it comes with most Linux distributions by default, and also Mac OS X (citation needed ;) ). This is a lot less hassle for Windows users (language is compiled into the executable, no other downloads required).
If mono runs on Mac OS X then surely that is a good platform as well?