Multi-user systems seems to cause a lot of problems for reasons which confuse me slightly, but that's coming from the background of developing under Linux where you're basically forced to accept that the user's home directory is the only place on the disk you have any permissions to write. I would have thought that the situation was similar under OS X, but apparently it's a little more permissive - certainly on my OS X system I don't seem to need any root passwords to write to the apps folder (which I don't really agree with, but I guess I can see where they're coming from).
Personally I'd like a read-only system partition, and a per-user overlay mounted via unionfs, but that's probably why freaky people like me don't design Linux distros.
If I can meekly offer a suggestion, BW, maybe it's worth finding whichever OS-specific functions you need to grab a user-specific home directory and hiding them behind an OS-independent abstraction - then you can use that directory for all data, and regard everywhere else as read-only. Of course, users will need to copy save games to share them, but that doesn't seem unreasonable.
This might well be the way it already works, and I appreciate that this is just one of the myriad pitalls of writing cross-platform code (even in supposedly platform-agnostic languages). Still, the multi-user one seems (naively) to be a relatively easy one to address, and well worth it for the potential hassles on non-Windows systems.
As a friendly aside to Chortick, it's worth trying to be a bit good-natured with your posts, and avoid rants and fulminations. Remember that there are fans on this forum who might well be able to help you out with some problems, but may well choose not to if you're not polite. Problems are frustrating, but keeping your frustrations to yourself is probably the quickest way to get a helpful response. Just in case it wasn't absolutely clear, by the way, I'm just a fan myself - I make absolutely no pretense at all of representing the opinions of Basilisk Games in things like this, just a bit of friendly advice.
In any case, I hope you manage to sort your problem out.