Thank you for the guide.

What I did:

1. Download latest firmware from asus (TF101)
2. Unzip using ES File Explorer (you will have another zip)
3. Unzip blob from extracted zip to /sdcard
4. Download Terminal Emulator
5. Open Terminal Emulator and switch to root
6. Ran the dd-command:
dd if=/sdcard/blob of=/dev/block/mmcblk0p4
7. Ran the sync-command (not required as it only syncs filesystems and no block devices, but it feels good to do)
8. Wait two minutes (should not be needed either)
9. Shutdown the device
10. Boot the device. you will see a progress bar.

Depending on the ROM you previously used, the transformer might be able to boot. If it does not boot, don't worry:
- Hold down the minus-volume button (down) when booting
- Wait till the message about cold-boot appears
- Re-press Down to switch the icon from Android to 'WIPE DATA'-icon.
- Press Up and Press Up again to confirm.

Even if your transformer did boot fine, without wiping, I do recommend a factory reset (via method above or via settings).

Be aware that this will erase all your data, also on the internal sdcard. External SD-card will not be touched.

For me it worked fine, now I have a unrooted device with original recovery (no CWM), Serial number starts with: BCO (yes a C ..)

Small Note about my config:
TF101-model (WIFI-only)
Original device updated to via OTA
Updated device to update of TF101G to get root
Rooted my device via RazorClaw
Installed CWM via Recovery Installer
Installed various roms (Revolution HD with Super Wipe to EXT4, Revolver)
Now back to stock ( via guide above, downloaded from asus)
For some reason, after flashing Spark (ICS from paulburton), my previously created nandroid backup did not work after restore. So, as I was not able to execute the above procecure, I did it via adb.

Extract the blob file from the zip file inside the zip file you download from the asus website (at the time of writing ~250 MB zipped):
- Place the blob file in the same directory as 'adb'
- Mount the /sdcard via CWM (volume keys + power button)
- Use the following command to put the blob file on the sdcard:
adb push blob /sdcard/blob
It will give you the speed and bytes transferred when it's finished, something like this:
1164 KB/s (546204969 bytes in 458.012s)
then you can start a shell to your device:
adb shell
This will give you a prompt. Now you can enter the dd-command above:
dd if=/sdcard/blob of=/dev/block/mmcblk0p4
When finished, you will see something similar to:
~ # dd if=/sdcard/blob of=/dev/block/mmcblk0p4
dd if=/sdcard/blob of=/dev/block/mmcblk0p4
1066806+1 records in
1066806+1 records out
546204969 bytes (520.9MB) copied, 172.216037 seconds, 3.0MB/s
If you want you can run the sync command, but again, it should not be needed but it does not hurt:
sync; sync
When this is done, you can exit the shell and reboot the transformer using the reboot option of CMW.

Now you should be back in business, atleast I was.