Digging up and old thread i know but im sure there are still some people lost.
If you have a rooted 1.57 w/ a custom rom, here are the steps to update:
Download the 1.83 SPF from Xda (use the forum search)
Use RSDLite to flash the SPF (once again, tutorial in XDA)
Congratulations, your on 1.83 stock.
To get root back just run gingerbreak again on your phone. Same way you did before.
You cannot flash 1.83 through CWM.
If you try pushing the update while you have custom rom, you will get android with the !
Only way to update is to flash the SPF using RSDLite.
Beware:
Move all apps off of the Media Area before you flash the SPF. When you use gingerbreak, it will wipe all apps off of the media area. Just got to settings > manage applications > (click the media area tab) > and just move all the apps back to phone. You can put them back after you get your phone updated and rooted.
If you have any questions, there is a much more detailed guide on the XDA forums for atrix.
Hope this helps for people that were trying to update.