Prior to my trip last week, I went in search of tooth powder. Obviously, my results are geographic specific, but for what is worth:

-- Found "Eco-Dent" tooth powder at Whole Foods and a health food store (County Sun, Palo Alto, CA)

-- Asked at each of the following and was told they do not carry tooth powder: Long's Drugs, Walgreen's Drugs, Safeway.

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Addendum:

The Eco-Dent (Lemon-Lime flavor [!]) came in a dark blue, nearly opaque plastic bottle with a top on it that looks like what might be found on a sqeeze bottle of lotion. So on my first trip through Security with it, I had it in a Ziplock bag along with a solid stick deodorant and ChapStick (since at the time it was still not clear if these non-liquids/non-gels were acceptable). Anyway, the TSAer at the conveyer belt looked at all the items (the tooth powder in particular), said "no problem", and told the person watching the x-ray it was OK.

On the return flight, put everything inside my carry-on, and no questions raised.