Ok, just to share my thoughts on this update: I did an update this morning (original firmware was the 2010.02-8 firmware). After making sure my rootfs was clean enough - and verifying it still (re-)booted correctly - I started the update.
The update itself seemed to go OK. Upon reflash:
* all fonts where replaced by [] (blocks)
* theming was totally gone (I saw a basic gray window with [] where the pincode window would be) but the button layout was largely the same
* upon loading X, in the left top corner a purple status bar was displayed, with no text/icons
* after a minute or so, the device rebooted itself

I also tried some other things, by e.g. using the camera button within that 'working minute'. This worked: the camera window appeared.
At one time even the browser appeared, but with a blank screen. In both cases it still rebooted. It thus seemed a GUI-restricted issue.

I reflashed the device using flasher3.5 (to the latest 1.2 update/Global ed.) and it worked.

Now I will have to restore all my settings; this is not for repetition and thus this will probably be my last update for the N900 (unless I reserve 2 weeks of my spare time to do the update).

(luckily I make a full / and /home backup each night, so with a lot of tweaking I prolly can get back a lot of my settings - let this be a warning however).