"For internal development you have a wide variety of options. Java is quite good and very popular among those who consider the option of not running their wares under Windows. The only way Mono can appeal to Windows-only houses is by making it possible to run unmodified (and proprietary) products that run under Windows on other platforms."
I'm working at a Windows-only house with internally built .NET apps. Mono is definitely part of our long term plans to run these apps on other platforms.
Congratulations. You are the case where Mono makes sense - you can make sure your apps run the same on Mono and on .NET.
Unfortunately, some shops need to use off-the-shelf software that may even run on Mono but is not officially supported and, if something did go wrong, they would on their own.
I'm working at a Windows-only house with internally built .NET apps. Mono is definitely part of our long term plans to run these apps on other platforms.