My latest CVS update has initrd support in it. I decided to see what it
actually took and it wasn't much. I just had to read the image in and tell
the driver where it is.
So, the thing to do is put 'initrd=<image>' on the UML command line. I've
also been putting 'root=/dev/initrd' as well, but I'm not sure what the right
thing there is.
If you don't want to pull CVS or the latest patch, then this will be in my
2.4.4 release, and probably in an ac tree pretty soon.