OK, I have a factory-fresh M54455EVB. I booted the box as-is (using "bootm 0" to launch the kernel), and everything ran fine. I then installed the new M54455 LTIB (2010-09-19) and got that set up on our 64-bit Ubuntu 11.04 box. (Took some hacking to do... had to install RPM, edit sudoers, and edit Ltibutils.pm to have it locate libm.so and libz.so correctly.) I changed the configuration from M54451EVB to M54455EVB, used the settings that were there, and added a few packages (iptables, dropbear, lszrz, etc). I had a uImage and rootfs.jffs2 built in about 15 minutes.
I then followed the instructions in "ColdFire M54455EVB BSP User's Manual, Rev 1.3" on how to upgrade the U-Boot image and then flash a new kernel and RFS to the device, no trouble. I updated the bootargs to include an explicit console, saved that, and then booted the kernel.
The kernel booted quite quickly, spitting out a bunch of new stuff about testing DES, DES3, AES, MD5, SHA1, etc. It registered usbcore, usbhid, ALSA, TCP cubic, protocol families 17 and 15, UDP, and TCP. Then it just sat there. Several seconds later, it mounted root (jffs2). A couple of minutes later, it started BusyBox v1.11.2. Over the course of the next 30 minutes, it went through about 25 more lines of bootup sequence. Right now, it's generating the 1024-bit RSA key for dropbear. It might be done by morning.
For whatever reason, right around the moment it begins mounting the root filesystem, the system slows down by factor 100 or more. It's still running... I'm confident that the RSA key will eventually be generated, even if it takes a few days.
Any idea why the kernel would suddenly slow down like that, especially given the fact that the old U-Boot and old kernel (flashed at the factory) booted and ran just fine?