As posted yesterday, maemo (great) community have managed to set up CPU clock speed over 600Mhz, let's see how to do it in a few simplified steps :

1- Make sure your battery is fully charged !

2- Download one of those modded Kernels (choose the speed you want to use) :

125 - 900mhz kernel. (Tested to give best results in terms of speed for a normal usage situation).

125 - 825mhz kernel.

125 - 800mhz kernel.

125 - 750mhz kernel.

125 - 700mhz kernel. (Probably the safest to try if your only looking for a slight speed increase)

125 - 600mhz kernel.

125 - 930mhz , 500mhz DSP (stock 430mhz DSP)

Or download the full package over here !

3- Make sure you put the kernel file into the maemo flasher folder on your pc so the flasher can find the file, in windows it's usually at C:\Program Files\maemo\flasher-3.5

4- Set up your N900 to flashing mode :

+ Switch off the N900
+ Press and hold down the 'u' key
+ Connect a USB cable between the N900 and host PC while holding down the 'u' key
+ When the USB cable is connected, it powers the N900 and sets it to flashing mode
+ When the USB icon appears in the upper right-hand corner, the N900 is in flashing mode
+ Release the 'u' key
.
Ps : more about flashing procedure over here

5- Run maemo flasher as admin (Still on windows) then run the command :

flasher-3.5.exe -k zImage-900mhz -f -R

(Of course, note that you'll have to change the zimage-900mhz to whatever kernel your using ie zimage-800mhz or zimage-700mhz)

6- it should take approx 10 seconds max, you're done !

By using the custom overclocked kernels, you agree that you take full responsibility if there is any damage caused by the flash.

In most cases, the N900 will not die and should be at least able to be reflashed to the stock kernel if it is unstable overclocked.

These modded Kernels will make the about page say unknown for the hardware and software, so personally, i'll wait for the PR1.2 before overclocking mine :) 

Update :

You'll just need to do some additional steps to bring it back so that you can still normally use automatic firmware updates in the future:

- launch X Terminal
- type sudo gainroot (you need the rootsh package installed)
- type apt-get install --reinstall mp-fremantle-generic-pr
- if asked for confirmation press Y and Enter
- when done reboot the N900