This may be defined elsewhere, but I thought it would be helpful to mention it here. Basically, I've been trying to rundown the usbnet issues for OpenSlug and was adding some printk statements to the kernel for debugging. To perform this all I needed to do was:

• Make some changes in the tmp/work/ tree... In my case this was editing the source files in tmp/work/openslug-kernel-2.6.9-r1/linux-2.6.9/net/ipv4/...
• Invalidate the timestamp files (by removing them): rm -f tmp/stamps/openslug-kernel-2.6.9-r1.do_compile
• bb openslug-image

This rebuilds the openslug-kernel from the compile stage and all the modules, ipkgs, and openslug-flashimage

This technique could be used for other stages also, like configure, or unpack, etc...

[jbowler: There are (now) more detailed instructions in ChangingTheOEBuild, this details how to avoid temporary changes being overwritten after a bk pull updates the openembedded directory tree.]