Thanks, David and Matt, Matt, yes, <condition> was what I was looking for - for some reason I thought you coudl not use them outside of <target>s, and, thus, could not use to define global variables. But since it is working great - that's all I needed.
David, thanks for extensive suggested options - they all look good. I did want to get away from splitting all my targets into dev/release versions though, just to be able to use different property values. So, the <condition> approach worked great for me. Thanks! Marina