Well, it isn't actually. The nightly builds are created on 10.4 and therefore run only on 10.4. The final builds are created on a really old iMac next to me on 10.2 to be able to run them on 10.2.8 and any higher version of OSX.
Regrettably, 10.2 isn't delivered with zlib, which is needed for png-encoding. Therefore, one of our libraries (ffmpeg) is compiled without zlib-support and you're unable to take screenshots. We keep working on this, but we're currently a bit out of ideas how to link zlib statically with ffmpeg, which is quite difficult because of ffmpeg's strange way of configuring itself.
If any forum user got an idea about this, we'd be glad to hear it
Edit: if we don't have a solution for this until Saturday, I fear that the 0.8.4a release will ship with disabled png-support. It is going to return in 0.8.5, since we are dropping 10.2-support after 0.8.4. ffmpeg compiles finely on 10.3 with zlib-support.