Andy, this old poll had 55 fmSoup members answer about their preferred platform for FileMaker development. Roughly 9 out of 10 prefer mac.
When going to DevCon, people and speakers have mostly macs. From there, I think there are 2 things that give this snowball effect: most tools we see surfacing come from a developer building the tool for themselves and making it available to a larger crowd with the least effort. If someone wants to commercialize a product, they will try having something crossplatform, but if this is somewhat challenging, it could very well be that it will be done only for one platform instead and that is influenced by 2 things I can think of, technical limitations of a platform over another and reaching the largest number of developers on the platform they use.
I develop on mac. Clients mostly use Windows machines. Client will report bugs they see on Windows, but if it is something that would only impact a developer, they won't report that.
Interestingly, with the webviewer layout object, there is also a great deal of disparity between windows and mac. Now that we can expect more addons to use interract with webviewers, I'm glad to hear that Claris is working on putting Edge in the webviewer for
windows instead of IE.