Sorry if this is obvious, but I haven't found a definitive answer in the documentation or in the forums. If I build a plugin with the CS3 SDK (either on Windows or OS X using Xcode), will it work in earlier versions of PS? How far back? Is the CS2 SDK more compatible with earlier versions?
Plug-ins compiled with the CS3 SDK should work fine under earlier Photoshop versions for the most part. The features and suites you use will determine how far back your compatibility goes. You can usually figure out from the documentation and header files when various features were introduced. (e.g. for filter plug-ins, the filter record fields are grouped in chronological order.) Just be sure not to use any new features of CS3. (As for what they are exactly, I guess that's for you to figure out. 🙂 The easiest way to ensure compatibility is to use the earliest version of Photoshop you want to support as your development environment; that way, any unsupported callbacks you try and use will return error codes and you will know not to use them.
Matthew's answer applies to Windows plugins. On OS X, the short answer is if you build a Mach-O universal (PPC+Intel) plugin for CS3, CS2 and earlier versions won't load it.
Unlike the single flavour of Win32 plugin, there are at least 5 plugin executable formats supported on the Mac, spanning 3 processor architectures and 15 years of operating systems from System 6 through OS X. I'm not sure if Adobe publishes a clear compatibility matrix, but you can ask here for more specifics.
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