I am running Ubuntu 10.04 and trying to compile a new uboot for nsa320. Compiling a kernel works great but the same proceedure for uboot gives no errors but does not produce a usable u-boot.kwb. For kernel install build-essential & gcc-arm-linux-gnueabi- and to build
ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- make uImage
works but
ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- make u-boot.kwb
does not. What am I missing?

What toolchain are you using? Is it the Ubuntu one or something else? I don't use Ubuntu. So I've never used their toolchain. But I have used CodeSourcery to cross-compile u-boot and it worked fine. AFAIK Ubuntu uses the Linaro optimizations in their toolchain. Those may be causing the problem. If you get no compiler errors then you may want to try changing the compiler flags to something more compatible like armv5.

You might also consider compiling u-boot natively. That's how I always do it because it is so small and takes little time to compile.

Yes Ubuntu uses the Linaro and I tried that first. Then I tried CodeSourcery to cross-compile u-boot and that also failed. Then I tried ELDK and that worked. I have no doubt that I am doing something wrong because I can cross-compile a kernel with Linaro and it works great. Just not for u-boot.