User-mode Linux is significant in a number of ways. From a theoretical perspective, it has demonstrated that the Linux system call interface is sufficient to implement itself. From a somewhat more practical perspective, the actual implementation has showed areas where the current Linux functionality is a bit lacking. These include the ability for ptrace to change system call numbers, the ability to manipulate address spaces, and for a process to intercept its own system calls. Fixing these will not only improve the performance (and on some platforms, the existence, in the case of ptrace) of UML, they will also help other applications. Fixing ptrace on i386 has made several other applications possible, and this may happen again with the other changes in the generic kernel that UML needs.
It provides a significant capability that previously didn't exist, which is making a number of interesting applications possible for Linux. The kernel debugging capabilities could speed up the overall development of Linux, and the gcov support may allow the development of coverage test suites which would allow the code to be exhaustively tested.
The possibilities for UML in the hosting industry may make it more popular than it already is and move it into segments where it is currently not popular. It is also possible that it will open up opportunities that didn't exist before, giving Linux ownership of those areas by default.
The wildcard application is the use of UML as a general purpose development platform. It may come to nothing, or it may be the killer app for UML. If it is the killer app, then Linux, in the form of UML, would become more prevalent than it is today, even on non-Linux platforms. This in turn would drive more changes in the code as userspace developers and kernel developers cooperate on the same code base. The presence of this as a significant platform in the industry would inevitably have repercussions on other platforms.
UML has a great deal of as-yet unrealized potential, and it will be exciting if it actually realizes that potential.