PAR for the Course: Working with the new pixel aspect ratios in CS4 by Chris Meyer is available to subscribers of the ProVideo Coalition's PVC Post Production Pipeline Newsletter. Others have to wait awhile. Here's an excerpt:
"Over the years, I have seen a lot of folklore and bad math employed to determine how to work with non-square pixels, resulting in a plethora of incorrect working practices. Therefore, in this article I'm going to spend a lot of time laying out the historical and mathematical basis for where these numbers came from. Hopefully this will provide you with a solid foundation on which you can build a new set of working practices. "
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Update: you can now be re-dorkulated without subscribing at PAR for the Course.