You're lucky: I also do desktop software but we can't afford to ignore the growing number of Mac users. While they still represent only about 8% of all computer purchases, they account for nearly half of "customers who pay for software" and C# isn't an option for us although we keep monitoring Mono's progress.

Also, allow me to disagree with you on something:

"If you have a small, well-scoped application, like your RSS daemon, C++ is going to be best of breed in most cases. Similarly, if performance is your absolute primary concern, there's nothing out there that can compete. For most desktop software, though, neither of these things are true."

Perhaps it's our difference in backgrounds, but most desktop software is more like that: small pieces that need small downloads, small memory footprints and instantaneous startup times. Just count the number of executable files on your hard drive and see what percentage of them eats more than 3MB of RAM (measured in 'private bytes'). Or you can look at the list of running processes: you'll see perhaps 1-3 behemoths like Firefox or Photoshop there, and one can only wonder how FireFox memory consumption would look like under JVM or .NET VM.