There is now a second Linux port of UML. Chris Emerson ported it to Linux/ppc. As of this writing, it is nearly completely functional, except for a bug in its signal delivery which prevents it from booting up to multi-user mode. However, as long as UML stays in single-user mode, pretty much everything else seems to work fine. UML/ppc is completely merged into the UML pool, so it's completely up-to-date as far as UML features are concerned.
This has special significance as the first architecture port of UML. It has provided the information needed to cleanly separate the architecture-dependent code from the architecture-independent code. There turns out to be not very much of it. Most of the differences are due to machine details which show through the ptrace interface. These include register names and the storage of system call numbers, arguments, and return values in the register set. There are also a variety of small differences between architectures which need to be accommodated: