The original Logo.bin is actually a 1.7 MB file which is two images in one file. When you are locked the device reads the top portion of the image only and thus the M is shown, when you unlock the device it selects the bottom image in place of the M thus displaying the nag screen.
The solution is very simple. Create your own image or use the stock one provided below.
The image needs to be 540 x 540 and in linux (gimp) save your image as a raw file and that is all you need to do. When finished with your image just flash to your device using the following command
fastboot flash logo (logoname.img)
File can be downloaded here ----> http://aosp.us/?p=260
That is all you need and you are all done.