Kholk, a senior member at XDA Forums, is able to successfully unleash 1.5 GHz on his Atrix 4G. He has released an Overclock Kernel Module for this. The developer says that he has done it successfully and is asking testers to test this. But we also like to point here that lots of doubts are going on in the XDA Forum and some users have even pointed out the plausibility of this Kernel Module. Still you are interested in taking the risk; you can proceed with below mentioned guide.

Pre-requisites Needed for Over Clocking
1. Your Atrix phone should be rooted. Follow this [Tutorial] for it.
2. PC
3. SD Card
4. Your phone should have any terminal app like terminal emulator installed.
5. SetCPU. If you do not have it, you can purchase it from Android market

Step-by-Step Instructions to Overclock Motorola Atrix 4G To 1.5 GHz
1. Download Compiled Module from http://forum.xda-developers.com/showthread.php?t=1010468 . If you are unable to download, just comment here and we will email the files to you. We can also send you the source code if you want.
2. Connect your device to PC via USB cable.
3. Put it on the root of your SD card.
4. On your ROOTED device, run a terminal emulator and type the below written commands:

su
cp /sdcard/olympus_overclock.ko /data/
chmod 777 /data/olympus_overclock.ko
cd /data
insmod olympus_overclock.ko

5. Launch SetCPU and follow the on-screen instructions to overclock your Atrix 4G.

Please note that this shouldn’t be treated as a kernel. It’s not a kernel, infact it is a kernel module.

You can post your any further queries in the above thread.

Disclaimer: Do it at your own risk.