I did this and it totally worked. Thanks! However, the upgrade process was iffy and a bunch of apps were freezing, so I had to wipe data, but you must do this *after* you already have root in ICS, otherwise wiping data will kill your su backup (I think).
So to summarize:
I am coming from the OTA 219.
1) Unfroze all the stock Motorola/VZW bloatware (obviously only need to do this if you've frozen it to begin with)
2) Backup data with MyBackup Pro (specifically things like text messages and call logs)
3) Install OTA Rootkeeper, back up root, temp unroot, then restore root. I didn't reboot after each step, but as with everything else YMMV.
4) Download the .zip that's floating around, extract it to find another .zip, put this on your sdcard.
5) Go into the bootloader by rebooting while holding down both volume keys, then go down to recovery. Select apply update and select the zip on the sdcard.
6) Pray
7) After everything installs you should boot into ICS. The setup process here was very finicky for me and clicking the wrong thing would cause a hard freeze (you can force the D4 to hard reboot by holding down power and volume down for 15s). What you have to do here is somehow manage to run OTA rootkeeper and restore root. Make sure you have the superuser app and it works!
8) I had to go back to the recovery menu here and wipe data/cache in order to get my system stable, ymmv, but it seems like if you have a lot of data (especially a lot of text messages and custom settings) the data causes ICS to freak out.
9) Restore your data using mybackup pro.
This is what worked for me, now I am on the ICS leak with root.
