I wouldn't mind an iDevice simulator for Mac that doesn't require Xcode and special access from FileMaker, Inc., either. There are apps that will let you mirror the display of an actual iDevice to the Mac, but not that will simulate the device. I'd be happy with a window mode just within FileMaker that simulates the dimensions and features of a device.
One thing I'm starting to find useful when building FileMaker Go apps is a handful of scripts I put together for previewing the window size. The scripts in this file will adjust the current FileMaker window so the content dimensions (the inside of the window, not the outside) exactly match the available screen real estate on iPad and iPhone (up to iPhone 4).