I'm happy to report that I managed to make my Gumstix Overo Fire boot from the microSD card! After a few failed attempts as I mentioned here, I noticed an error message in the boot sequence:
Texas Instruments X-Loader 1.4.2 (Mar 27 2009 - 08:51:34)
Reading boot sector
Loading u-boot.bin from mmc
U-Boot 2010.06 (Sep 09 2010 - 13:32:51)
OMAP3530-GP ES3.1, CPU-OPP2, L3-165MHz, Max CPU Clock 600 mHz
Gumstix Overo board + LPDDR/NAND
DRAM: 128 MiB
NAND: 256 MiB
Board revision: 0
Tranceiver detected on mmc2
Die ID #6b2400040000000004032d460c005010
Hit any key to stop autoboot: 0
Unknown command 'mmcinit' - try 'help'
Booting from nand ...
NAND read: device 0 offset 0x280000, size 0x400000
4194304 bytes read: OK
## Booting kernel from Legacy Image at 82000000 ...
Image Name: Angstrom/2.6.29-rcfinal+r0+git90
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 2723228 Bytes = 2.6 MiB
Load Address: 80008000
Entry Point: 80008000
Verifying Checksum ... OK
Loading Kernel Image ... OK
Starting kernel ...
A Google search lead to this discussion suggesting to "clearing the environment in nand" with:
nand erase 240000 20000
Now I have the Gumstix Overo Fire booting from the microSD and even the Ethernet on the Tobi board is working (it didn't work with the factory image from 2009). I tried both the desktop and the console image and both seem to work. I am very impressed tby the fact that the console image only consumes 45 MB of RAM once it is booted, leaving almost 200 MB for applications.
Here is a short video showing the Gumstix Overo Fire boot from the microSD card.