alright porselinaheart..i'll try to help..hope i can remember the steps i did 2days ago...but i can't guarantee anything yea..

here goes..

first of all, please backup/copy your original v46..these are the files to be copied/backup as dpkg -i the new v46-wl12 will overwrite the original v46..

1. zImage-2.6.28-maemo46 (/boot)
2. vmlinuz-2.6.28.10power46 (/boot/multiboot)
3. 2.6.28.10power46 (/lib/modules)

then do these ORDERLY as per instructed in the README file, AND note the added dpkg for bootimg..

after you are done, FROM THE PC, extract the "zImage-2.6.28-maemo46-wl1" file from "wl1251-maemo\binary\kernel-power\kernel-power-bootimg_2.6.28-maemo46-wl1_armel\boot.."

copy the file to /boot ON THE PHONE

copy again into /boot/multiboot, and then rename the file to "vmlinuz-2.6.28.10power46-wl1" without the quotes

now, go to /etc/multiboot.d and create a new .item entry called "02-Maemo-2.6.28.10power46-wl1.item"

edit the file as follows :

now remember the original v46 that u backed up just now?copy each of the file to the respective folder..(refer the above for the path)

now make sure in /boot , u have
1. a folder called "multiboot"
2. zImage-2.6.28-maemo46
3. zImage-2.6.28-maemo46-wl1

in /boot/multiboot , u will have :
1. vmlinuz-2.6.28.10power46
2. vmlinuz-2.6.28.10power46-wl1
3. vmlinuz-2.6.28.NIT.06 (IF INSTALLED)
4. vmlinuz-2.6.28-omap1

in /etc/multiboot.d, you will have :
1. 00-Maemo-2.6.28-omap1.item
2. 01-Maemo-kernel-power46.item
3. 02-Maemo-2.6.28.10power46-wl1.item
4. 11-NITDroid-mmcblk0p5-2.6.28.NIT.06.item (IF INSTALLED)

in /lib/modules, you should have :
1. a folder called "2.6.28.10power46"
2. a folder called "2.6.28.10power46-wl1"
3. a folder called "2.6.28.NIT.06"
4. a folder called "2.6.28-omap1"
5. a folder called "current"

well, basically, these files are in my N900, and it's working..i can boot either v46-wl1 or v46..both are in working state..no issues up till now

erm..i guessed this is what i did..i'm not sure if i missed a step or two..but try it first..