Interesting! Now I realize it IS a server problem at Nokia. Using the link http://nds2.fds-fire.nokia.com/fdp/i...ED_MR0_ARM.bin
with either windows or wget under linux the download is prematurely cut off from the server side at around 32 MB (reports 206 Partial Content). So what the hell is going on with this server?
Dammit! This must be reported to Nokia, this is the actual problem. Are there any mirrors for this very same file that WORKS?

EDIT: Downloaded the image from http://tablets-dev.nokia.com/nokia_N900.php
I just hope it's the same that the updater uses (for me, the global image should be the right one). I placed it in the updaters folder, I finally found it. Since I'm an XP user, the folder path for me was:
"C:\Documents and Settings\All Users\Application Data\Nokia\Nokia Service Layer\A\nsl_service_module_00001\vpls\www.dsut.onl ine.nokia.com.oti.caresuite\Products\rx-51"

Hope it help! Updating now with NSU!
(Nokia: Get your sh*t together!)