Different types of crossbuilding: http://en.opensuse.org/Build_Service/Concepts/CrossDevelopment#Types_of_Crossbuild
A bit terminology:
The default package list for a build environment is defined in the prjconf (see http://en.opensuse.org/Build_Service/prjconf) :
Preinstall + Required + Support + Packages from dependency resolution
For the "native"+emulation cross-build (as in: without cross-compilers), we use qemu-arm. The callenge is now to keep the environment but add speed through cross-compilers (="cross"+emulation).
With all changes in place, we reach really good speed. E.g. for:
Build time in minutes
|x86 native||armv5tel native||armv5tel cross||factor x86->arm native||factor x86->arm cross|