Since I'm on bleeding edge FreeBSD (8-CURRENT), I'm often rebuilding my world quite often. Whenever i interrupted the build processed, I tried to continue it with

make buildworld

I now found out that this is quite stupid! "make buildworld" cleans the /usr/obj directory and rebuilds everything. This is a good thing, if you updated your sources. But if you only interrupted your build, you can safely go on with the build (at least i think so ;-). You can tell the build process to not clean the object directory by using

make buildworld NO_CLEAN=YES