Marc Aurelien over at webOS France recently testing overclocking and battery life on the Touchpad. The take-away is that you get the best battery life at 1.2GHz, the clock speed set by HP. Downclocking really doesn't do much. The savings in battery life are not worth losing performance. While overclocking will increase performance, if you run your Touchpad at the highest speed currently achievable, 1.8GHz, you will lose around 16% in terms of battery life.
The rest is a pretty interesting read, if you can read French (and I guess Google Translate will work well enough).
Edito : Autonomie VS Overclocking : le test complet