Oh boy, the beginnings of an OS X debate!
There are several reasons why supporting OS X 10.5.x would be counter productive in terms of adding new features to the viewer, and trying to support an OS where graphics driver quality is laughable at best.
Supporting OS X 10.5 would put us in a bit of a predicament in the event I ever want to add some features that make use of newer APIs on OS X 10.7, such as fullscreen support.
Many of these features can be implemented while easily maintaining backwards compatibility with OS X 10.6, but 10.5 wouldn't be as easy to support in this case.
There's also the issue of graphics drivers. 10.5.x's graphics drivers (10.5.8 included!) are notoriously bad. Apple made advancements in fixing many issues in this category to make OS X into a slightly more viable platform for games in the OS X 10.6 release cycle, and even more advancements in OS X 10.7's cycle such as the addition of the OpenGL Core profile for OpenGL 3.2.
Now, another reason why we won't be supporting 10.5, in part has to do with Apple's SDKs. Exodus is compiled with a slightly modified version of Xcode 4.2, that has GCC 4.0 support to work around a bug when compiling with GCC 4.2 or LLVM-GCC 4.2. On top of that, we compile against OS X 10.6's version of APR, due to another bug in the viewer that arises when compiling with LL's version of APR that prevents local textures from loading (which means you basically see gray blocks for the UI, and other seemingly unloaded textures in the viewer). The end result is, the viewer tends to be unworkable under OS X 10.5, in the vast majority of cases simply crashing when it launches.
So, will we be fixing it for 10.5.8? No. Apple has made it pretty cheap, and incredibly easy, for users to upgrade to OS X 10.7, and most intel-based Macs support 10.7 anyways.
If you chose not to upgrade to OS X 10.6 (therefore meaning no access to the Mac App Store, and by extension the ability to purchase OS X 10.7 at a cheaper rate), then you can purchase a thumb drive with OS X 10.7 on it directly from Apple here:
http://store.apple.com/us/product/MD256Z/A?
OS X 10.7 has seen excellent performance gains for 3D applications (even if they're still not quite on par with Windows), and the crash rate tends to be generally much lower in comparison to OS X 10.5 and 10.6 when it comes to applications in general. There really isn't much of a reason not to upgrade to 10.7.