By popular demand, here's an app to set undervolt-values. It works on any ROM which runs on a suitable kernel, but needs init.d-support to be able to keep settings between reboots. (If your device supports it you'll get a button you can use, if not a little message in its place.)
WARNING: By using this app, you're playing with fire. It's in fact very likely you'll lose data if trying to see "how low you can go". Do not experiment without backing up everything first.
It's easy to use, and easy to freeze your phone with, make sure you know what you're doing. (Yes, changes become effective as soon as you apply them.)
It's clever when it comes to the volt_scheduler script as well, it only edits the line which sets the values at startup, meaning you can change the script as much as you like, the app won't touch anything else in it. (It also fixes the script-permissions on Topogigi 1.9, which aren't set correctly on a default install.)
The app will NOT let you change the startup-script until you have applied the settings. This is an attempt to avoid applying bad undervolts at startup. (The theory is that your phone will freeze if you're undervolting too much, and hence you won't be able to save those values to the startup script. If you try hard enough however, you will be able to save unstable settings, with the possible consequence of boot-loops and data-loss. I'm not kidding, be careful!)
NOTE: If you're using a system-monitor (System Tuner for example) and it doesn't show the voltages you applied, you need to force-stop and restart it before it'll show the current values (pun intended.)
NOTE2: Tegra 2 never supplies less than 770mV to the CPUs. The app will let you set undervolt below 770 though, and will in brackets show you what the real undervolt (on Tegra2) actually is. (Until recently I thought this was a hardware limit, but learned it is a kernel-limit. There may be kernels out there which can go lower. If the minimum frequency's default voltage is not the same as the kernel's minimum, the numbers in brackets will be off by the difference between the two.)
HEEELP: "I was very stupid and saved the undervolt-values to the startup script without testing properly first. Now it keeps rebooting. Is there anything I can do?" See Post #2.