Plugins built with CS3 SDK compatible with earlier versions?

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?

Thanks,

Not so very obvious, then? Nobody has tried this?

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.