The web page below contains interesting info about what versions of InDesign (CS, CS2, CS3, CS4 and CS5) are being used.
Amongst many other things: it looks like around 44% of the Mac user base has upgraded to CS5, whereas only about 34% of the Windows user base has done the same. Are Mac users more progressive?
http://rorohiko.blogspot.com/2011/03/what-version-of-indesign-do-you-use.html
Have a look, let us know what you think!