I currently have CS5.5 and collaborate with somebody who is using CS5. I am just wondering before I upgrade to CS6 whether idml files from InDesign CS6 will still work in the same way going back to CS5.
A definitive answer from somebody at Adobe would be much appreciated.
We're pretty much all just users like you here, but that doesn't change the answer, which is "yes."
More probability of text reflow, however, plus more new features that won't translate, and you'll have problems if you place pages from a CS5.r or CS6 file in your document.
Yes, with the same caveats about new features, text flow, etc.