So this is how things went wrong:

My phone was working fine 3 days ago. After installing some apps, including backupmenu and uboot, I rebooted. After that I cannot boot into maemo anymore. The phone went into reboot loop when I tried to boot "attached kernel".

So I thought maybe something was wrong with the kernel. What I've tried so far:

1/Reflash the kernel only.
2/Reflash rootfs with pr 1.3.1 firmware.
3/Reflash rootfs with pr 1.3 firmware.
4/Reflash both EMMC and rootfs.
All done in Ubuntu. Flashing attemps returned no error. But didn't work.

I've also tried to fsck the partitions in ubuntu 12.10 (first I tried this, but mass storage didn't work, then I tried this and it worked). First it returned some errors about bad blocks, but after running fsck -af a few times it returned no more error. I also formatted the partitions using mkfs.vfat /dev/sdb1 and mkfs.ext3 /dev/sdb2 (n900 is mounted in /dev/sdb)

Battery was well charged (using charging script in rescueOS).

After doing all the stuff above I flashed both EMMC and rootfs once again, but it still didn't work.

The booting behavior is the same: nokia screen, then 5 dots running back and forth for a while, and then shutdown.

I'm already running out of idea what to try next. I've always thought reflashing both EMMC and rootfs can solve everything. Now apparently it's not true in my case.

Any idea or advice for me? I've been struggling with my N900 almost for the whole past 2 days already.