I can confirm that upgrading uboot to the version found at http://plugcomputer.org/index.php/us/resources/downloads?func=select&id=15 allows me to boot custom kernels.

You have described pretty much exactly what I have been doing. I load the kernel from tftp, manually set bootargs (I have been using a mircosd card for my rootfs), and then use bootm to boot the kernel. I use the same procedure to boot the stock kernel (replace tftp with reading from nand) and it works fine.

I have not tried updating uboot at all yet.

I am having this same problem. The only differences I see are that I am using version 2010q1-188 for my cross compiler, and I am loading the uImage off tftp initially. The only thing that I have gotten to work for me is the image already on the internal flash.

It looks like it may not be fully uncompressing the kernel? As appears to jump to booting the kernel much sooner.

Anyone have any thoughts on this?