You "updated" the config? You mean you compiled a native kernel, then reconfigured for uml?
If so, that might be the problem. Do a make mrproper and configure from scratch. Re-making for a differenct arch can leave old files lying around, which is probably why it's screwed up.
One thing to note is that with 2.6 you can now configure and build in a different dir. This is handy as it keeps the src dir clean. You need a clean src so do a make mrproper first. Then, e.g. 'make O=../build-um ARCH=um xconfig' etc.
Of course, you must specify the O=../whatever for each invocation of make. You can then build as many kernels as you like, building each in a different dir. (Note that you still need to run make from the kernel dir, not the dir you specify with O=).