I only have an OS X 10.8 machine with Xcode 4.6, so I can't target base SDKs 10.5 or 10.6 (for developers with 10.5/6), unfortunately.
Actually you can, but you would need to get the previous SDKs from older Xcode releases, then copy these SDKs next to the other ones (those from your latest Xcode installation).
Either it's a bit more complicated than that, or I'm doing it wrong
I downloaded Xcode 3.2.6, installed the 10.5 and 10.6 SDKs, copied them next to my 10.7 and 10.8 SDKs, and tried to build (using the command line utils provided by Xcode 4.6), and I got a ton of errors about standard headers. I'm guessing I have to build with the command line tools from 3.2.6, but I don't have a whole lot of time to mess around with that. If you've got tips, I'd appreciate them. Otherwise, I may not be able to build with the 10.5 or 10.6 SDKs (or if I do, it might take me forever to get to it).
I've published my build scripts on github here:
https://github.com/mjbshaw/SFML-OS-X-NightliesMy first builds can be found under the bin folder. I've tried to make the folders structured just like the official downloads.
If someone could please just take a peak at them and give me any kind of feedback, that would be great! If someone has questions (like exactly what the build settings are), feel free to ask. I will try to work on these a little bit more over the next couple of days whenever I get a free moment to get a better workflow and put a link on my blog.