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Most likely on November 2010, but sources have not confirmed any exact dates. Apple might release the 30" version first though, as it is the cheaper due to smaller touch panel. The 40" and 50" version will most likely be released on April 1st, 2011.

Or one could search for existing thread via MRoogle and find this:

iMac touch - make it happen!!! - thread on MacRumors Forums

Anyway, here are two articles,, being as accurate as it can get, they are from yesterday though, thus they might be already irrelevant.

Apple Discloses Methods for Transitioning Between Mouse-Based and Touch Interfaces - article on MacRumors first page
MacRumors Forums thread about the above mentioned article

Apple filing shows touch-screen iMac with adjustable stand - article on AppleInsider, an irrelevant Apple Rumour News page
 
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