I believe I have been hit by the same or a similar problem. I had been using 2012.04-1 with maemo's kernel, kp50 and ubuntu on the uSD.

Today I installed 2012.04-2:
1. installed tools and flasher *without* removing 2012.04-1;
2. changed item files: sed -i 's/REUSEATAGS/OMAPATAG/' *.item
3. ran u-boot-update-bootmenu
On reboot bootmenu shows fine, but neither maemo nor kp50 can boot maemo. Both poweroff the device after the 20s timeout. Ubuntu on uSD boots fine.

My mistake, I think to myself, and so I reflash the kernel. System boots, I purge tools and flasher, and install them again. Next bootmenu is updated and reboot, but no changes: maemo's kernel and kp50 bomb out, ubuntu boots fine.

While in ubuntu I fsck'd all mmc partitions, and found no errors. HW rev is 2204, if that matters.

I had to flash the kernel again, and to be sure, I purged 2012.04-2 debs and reinstalled 2012.04-1, and reverted the ATAGs changes, and... now everything works again.

So for my device there is a regression with version 2012.04-2.