I would like to test software designed to run on a Mac and would need a couple of OS versions (old ones, too) and maybe other Mac software to set up my (virtual) test machines.
For the same scenario on Windows, I can use the licenses available via TechNet or MSDN subscriptions. At the ADC Mac Dev Center, I couldn't find any specific information as to what is available. Does an ADC membership get me downloads of software / OS's to test against? If not, what is available?
Yes it does. You can download the operating systems as well as betas (although not as early as "proper" beta testers).
Bear in mind that I don't believe any version of Mac OS (barring OS X Server) is licensed for use in a VM. In theory, if you want to test different versions of the OS you'll need to do either multiple installs on a single system (My first iMac came with OS 9 and OS X installed this way, not sure you could get multiple OS X versions installed on a single box) or have lots of boxes.