Now that ARM support in the OBS is getting more mature, here a report on the Status of PowerPC and MIPS builds using QEMU. They are implemented similiar to the ARM solution, and use QEMU Usermode (to allow speedup with x86 based cross compilers like we do for ARM).
First of all, PowerPC native builds do work since a long time (3+ years). At the beginning, only XEN virtualization was available for OBS, and XEN did not work on PowerPC hardware. Recently, KVM autosetup was added to OBS with release 1.8. KVM also works on PowerPC machines, so there are now fully functional PowerPC native builds with virtual machine support available.
QEMU Usermode builds for PowerPC are working on 32bit targets. They had been tested on all linux distribution targets using 32bit PowerPC mode (all Debian or Ubuntu PowerPC have working builds). Due to the lack of some functions in QEMU, these builds do not work with QEMU inside a KVM virtual machine (the build results cannot be extracted due to a missing ioctl emulation on PowerPC). Since currently Fedora as well as openSUSE have dropped PowerPC support in their distros, this leaves only 32bit targets on Debian based packaging to be supported. Anyway, should someone need 64bit support, he can use a native machine to work with that.
QEMU Usermode builds for MIPS had also made the first beep inside OBS. They support currently Debian 4.0 mips and mipsel 32bit builds, and Debian 5.0 mips builds (mipsel currently fails on QEMU). It seems there is no RPM based distro available anywhere, so I had no chance to test this case. 64bit MIPS Usermode seems to be broken in QEMU, so it would need fixing. Also, QEMU Usermode hangs for MIPS builds when running in a KVM virtual machine.
A QEMU used for both the above cases is available now for quite a while in the OBS project openSUSE:Tools:MeeGo. The qemu package there is named qemu-deploy. The other small changes in osc, build and obs-server code needed are already in git master and will roll out with OBS 2.1.
In case you would like to help me enhance the support for PowerPC or MIPS and close missing parts (get MIPSEL working, fix KVM builds), feel free to contact me.