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I've been interested in this topic, and have read some anecdotal evidence that says about 160F is the ideal. A paper from the automotive section at LSU says most automatic transmission OEMs don't try for TC lock-up until above 120F (of course ours is about 150F).
The only factory spec on minimum temp I've come across is from the GM truck division which says NOT to operate with dexron III filled transmissions below -13F (-25C). They suggest a pre-heater or let it idle until the ATF reaches that temp. I'm presuming that the fluid is too thick below that point to effectively flow to all the proper places (not stated in their paper).
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