My question is already stated in the title -- following some more info how it came about (italics), and at the bottom a summary what I tried so far:

Probably, I was incredible stupid today. After increasing numbers of issues with the tracker, which couldn't be cured by the usual means, I made a tracker reset. No big deal as I saved everything to the MyDocs partition.

To save the SMS I used QTweakN9 (http://talk.maemo.org/showthread.php?t=80600), which I installed the first time. During the first start the "bash" gets installed, too. That reminded me to search if there's a package of my favorite shell "zsh" for the N9. Yes, there is (http://talk.maemo.org/showthread.php?t=55947) and I was happy as it worked really well. So happy, that I appended "exec /bin/zsh" to my ~/.profile -- changing the login shell is sometimes not such a good idea, so I used that workaround. (That was mistake no. 1 I suppose, but I'm not sure, that my problem has something to do with that or if that was just an accidental coincidence...)

However, everything worked well and after an hour everything was restored from tracker reset. Haven't done a backup to my PC quite long, so I thought that now would be a good time. I already had MyDocs opened in the explorer, but then I realised I didn't made a fresh backup with the internal N9 backup app. So, unplugged USB (mistake no. 2), and was about to start the backup, but the keyboard didn't appear to name the backup... crap, what's going on?! I already wanted to copy&paste some string, but than made a reboot instead (mistake no. 3) -- "it won't hurt". But it hurt badly! The N9 get stuck at the NOKIA logo -- nothing happened anymore. (So at that moment no boot-loop.) Damn!

I searched the internet and found some options I have:

• put the device in R&D mode with flasher (http://talk.maemo.org/showthread.php?t=85596), trying with "no-lifeguard-reset" set or cleared.
  => that enabled some fancy messages, but didn't help to boot the device.
  
• flash a rescue system (http://wiki.meego.com/N950/Fixing_MALF_state)
  => sounded very promising and device was recognized by my linux box:
  
  
  Code:

  [262423.057512] usb 1-8: Product: Nokia N9
  [262423.057514] usb 1-8: Manufacturer: Nokia
  [262423.057516] usb 1-8: SerialNumber: 357923046591014
  [262423.067414] scsi26 : usb-storage 1-8:1.0
  [262424.069077] scsi 26:0:0:0: Direct-Access     Nokia    Nokia N9         0316 PQ: 0 ANSI: 2
  [262424.071939] sd 26:0:0:0: Attached scsi generic sg9 type 0
  [262424.076530] sd 26:0:0:0: [sdh] Attached SCSI removable disk

  BUT: sdh didn't show up in /proc/partitions.
  According to this (german) site (http://opensmartpad.org/meego/howtos...ll-backup.html) that rescue system does not work with the 16GB version of the N9! *@@$$!!!
  
• tried the MOSLO kernel (http://talk.maemo.org/showpost.php?p...&postcount=457)
  => no luck either, but I tried this time with windows, as I never used networking via USB under linux.
  
• flashed the firmware (but not the MMC, I want my data alive!)
  => N9 gets stuck at the NOKIA logo, LED lits and then reboots again.
  

I have no more ideas, but I deeply hope some of the pros in this forum have another one.

Especially I'm interested, if somebody can confirm that the approach with the rescue initrd is working also with 16GB devices.

My phone is not SIM locked, but however I have MfE accounts setup (which can trigger the device lock, according to a page I did not find anymore).

Thanks a lot in advance!