Jakiman's N900 Overclocking Summary:
1. Overclock is done by flashing the modified kernel using maemo flasher.
2. No setting or data gets changed/deleted. So it is a transparent flash.
3. In stock form, N900 scales between 250Mhz and 600Mhz dynamically.
4. With Lehto's kernel, it scales between 125Mhz and xxxMhz dynamically.
5. So in theory, during idle, it may actually even use less battery.
6. In most normal everyday use, I do not expect any extra battery usage.
7. Flashing takes literraly about 1 second to do. Very quick and painless.
8. If something goes wrong, just reflash the stock kernel back.
9. So far, majority have reported theirs to work just fine up to 900Mhz.
10. 1ghz kernel is not available to public "yet" due to liability concerns.

Nokia Flasher – http://tablets-dev.nokia.com/maemo-d...-downloads.php
Lehto Kernels – http://www.saunalahti.fi/crailt/

General Steps:
1. Power off the N900 completely.
2. Hold "u" on the N900's keyboard, while holding, connect it to the PC via USB cable.
3. You will see usb icon on top right of white Nokia screen.
4. Now you can let go of "u" on the keyboard.
5. Now use flasher utility with the kernel file located in the same directory.

flasher-3.5 -k image_file_name -f -R

6. It should take about 1-2 seconds then it'll say Done.
7. Now your N900 will show white Nokia screen.
8. At this time, you can pull out the USB cable.