EDIT: this post is outdated. please goto http://wiki.maemo.org/Overclocking

I have uploaded two new low voltage kernels.
Both are the same as the maemo20 kernel in extras-devel except for the lower voltages,
i.e. they preserve your freq. limits over phone calls and don't lock to 600Mhz during phone calls.
The default limits are 250-600 MHz.
A new feature in all maemo20 kernels is the bq27x00_battery module by Matan
which gives you more info about the battery in /sys/class/power_supply/bq27200-0/

The ultralow voltage (ulv) kernel is inspired by Letho's settings.
The less extreme low voltage (lv) kernel is inspired by Matan's settings.

Download ULV:
Download LV:
Installation
replace the values (as root) for temporary change
or edit /etc/pmconfig for permanent changes of the frequencies.

To enable 125Mhz use 124999 for scaling_min_freq.
According to BlackDiamond you should
"echo 1 > /sys/devices/system/cpu/cpu0/cpufreq/ondemand/ignore_nice_load"
for 125MHz http://talk.maemo.org/showpost.php?p...&postcount=185

Also try the hint by nightfire http://talk.maemo.org/showpost.php?p...&postcount=170 for improved responsiveness.

The warning "WARNING: at /home/builder1/maemo-fremantle-armel-extras-devel/work/kernel-maemo-2.6.28/arch/arm/mach-omap2/clock34xx.c:443" some people see in dmesg indicates that we don't really know what we're doing
The same applies to the DSP settings - it is not clear whether Letho's and my kernel actually set them to >430MHz. Someone familiar with frequency selection should have a look at the code...

so far the ULV kernel works fine for me with [125,600] Mhz till 850 MHz. 900 crashed it. EDIT: I just increased the 900 voltage in an update
EDIT2: another update replaced 800 by 810MHz in ULV and LV kernels