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.
I guess it heavily depends on a market segment, but for "family computer software" it is true: lots of PC users playing with free/evaluation versions but for when it comes to premium subscriptions and premium features - most of them evaporate, whereas Mac users like to stick around.
Look at various PC/Mac softwares out there: most of PC-only software belongs to "free&shitty" category, but nearly everything for OSX you have to pay for, and they (users) are accustomed to it.
I am not sure if I can post results of the research my company has done (and paid for) here.
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.