Mono for Mac just needs more effort
The article misses the point in a couple of ways:
"The real stomping ground of Mono is undoubtedly Linux" Actually, it's open systems. You noticed that on Mac it works fine with all the standard, open APIs (meaning most non-GUI functions, plus GUIs under X) but not with the proprietary and Mac-specific Cocoa. You then derail completely: "any efforts expended in terms of building a more feature-complete implementation of Cocoa# would, in a sense, be wasted effort from a portability point of view since Cocoa" well, no, it wouldn't be wasted, because then Mac users would be happy to run Mono GUI apps, and you've already correctly observed that they won't run apps that don't look native. And "What’s the point in writing your application using an ECMA certified, platform-neutral language when all you’re doing is making calls to a proprietary, platform-specific toolkit?" okay, so let's give up writing in C, because all we'm doing is generating proprietary, plaform-specific instructions. Seriously, this is a major part of the point of Mono: you can write one app, and it will run on Windows (where it is simply calling proprietary, platform-specific APIs) or Linux (where it is calling platform-specific APIs that may be open, but certainly aren't as neatly standardised as Mono's), or whatever other system you're on.
And then this: "I expect some pundit will be along soon to assure me that if I just added a couple of lines of gobbledegook to this config file". Why didn't you ask whether Apple could make X work decently? A rootless X server that starts automatically when required would remove all the objections to running X apps under Aqua except for the "look-and-feel" one, which for many programs is enough.
Finally, to describe Cocoa as proprietary is not entirely fair, and you didn't mention the obvious riposte of the Mac-centric open source development, namely to use GNUStep. Read this article, for example:
Finally, your conclusion: "in terms of how/where they fit into Mac development, I think it’s time to step back and re-evaluate where we’re going." is nonsense. It fits in just like it does on any other desktop platform, and it's obvious where we're going: eventually, Mono will work fine on Mac OS. What seems to be lacking is sustained effort in that direction. All that's needed to get there quicker is more effort. Apple could help if they wanted, and, as for any minority platform, it would seem to be a big win: they're far more likely to attract than lose devotees by such a move. Equally, more Mac developers could get involved. Like most OSS projects, this one will stand or fall in the straightforward market-place of development investment, wherever it comes from.