I just checked. hw.acpi.acline gives "1" for when AC is available and "0" for when it is not. This does work correctly. I don't know where to look in linprocfs to know if it parrot's the setting correctly.
From my observation of the notebook in windows mode, I strongly suspect that the reason it sticks to 150Mhz mode on battery is that the battery system can't power the cards at higher frequencies.
Anyways... zander: thanks for looking at this --- it's one of the largest gnats in my enjoment of this laptop