Since UML is a full-blown Linux kernel, it provides a completely authentic Linux environment to its processes. This is not exactly a revolutionary concept, but it becomes interesting when UML is ported to operating systems other than Linux.
A number of commercial Unixes are starting to acquire various levels of Linux compatibility. However, the highest level of compatibility would be gained by actually running Linux on those other operating systems. This is what exactly what a UML port would do.
How much work it would be depends on the target. The other Unixes might be fairly easy porting targets because of their similarity to Linux. They would just need something equivalent to the Linux ptrace system call tracing mechanism and a few other things. More foreign operating systems such as Windows would obviously be harder, but they would also be interesting.
There has been some work done on a Windows port, but it is fairly preliminary at the time of writing. UML can boot up to the point of starting init, but that was accomplished by stubbing out a lot of code that will turn out to be important.