Ah, 412MHz is okay. (As to the 600+MHz capability claim, that was just extrapolating from some possible manufacturing methods. All the articles that claim it's spec'd to 600MHz are full of it. I doubt that Apple would pay for chips that tested that high, when it's not necessary.)
Other phones, with more battery capacity, commonly run at ~500MHz.
My four+ year old WM5 Samsung i730 ran at 520MHz, and I used a third party overclocking tool that boosted it over 700MHz for apps that I specified... and down to 200MHz for simpler apps. This allowed me to conserve battery while doing little things, but get the speed when I wanted it.
The i730, btw, had stereo speakers and surround sound. It also had a vertical slider keyboard that moved like velvet. It would be great if Palm manages that same feel in the Pre. Too bad the i730 had a god-awful monster antenna block sticking out. Oh well, memories.