I had the same problem as you.

I did:

Turn off phone
Connect phone to PC using USB
Copy all files off the drive that mounts - this is NOT your calendar, contacts, conversations etc - they are on a 2 gig hidden partition you cannot access this way, but this IS you music, movies etc off the internal 32 gig memory (well, more like 28)

Downloaded flasher, see: http://wiki.maemo.org/Updating_the_tablet_firmware
Downloaded the same PR1.1.1 firmware that may phone was on when it got stuck in the loop. These are also called FIASCO images.

Ran flasher telling to flash the PR1.1.1 to my phone
Connected the phone while holding U key on the phone
The computer stalled at "USB device not found, waiting" and I had to go into Vista device manager, and Reinstall Drivers for Nokia N900 Update Mode
As soon as the drivers repaired, the flasher wrote the new PR1.1.1

My phone booted OK, all apps are gone (this is expected)

I used the BACKUP command of the phone to make a backup onto the memory card, NOT the internal 32gig memory! If you dont have a memory card make the backup onto the internal memory then GO BACK to the first step above (phone off, connect to computer) and copy the BACKUPS folder off th drive that mounts, onto the desktop of the computer
Remember: your memory card is not available if you have the battery cover of the phone removed

Now i had my contacts etc backed up (using the BACKUP command of the phone) I used flasher again and flashed a new PR1.2 FIASCO/firmware, and I also flashed the eMMC (this means the contacts, sms etc are lost, which is why you backup them first, and you also lose all the data like movies, music etc in the MyDocs - but you already copied that to your PC in step 1)

Now boot your phone and restore the backup. Note: your backup contains an application list but it was made on a fresh flash of your firmwre so there are no apps. If you have no older backups where the app list was popualted you'll have to try and remember all the apps you had installed. If you have an old backup, restore the apps list out of that, and then App Manager will offer to reinstall all your apps for you..


My understanding is, some things changed around with PR1.2 which is why there is an eMMC image for it, but not for others (other than the old one) - because my phone was 1.1.1 I reflashed this same firmware because flashing 1.2 but having an old eMMC might have made the reboot loop continue.. Old firmware, unchanged emmc, do backup, new firmware, new emmc, restore backup
And I'm now ona 1.2 phone that's blasted clean and still has my sms, calendar, contacts etc