I just wanted to let you know that libc++ and compiler_rt and now dual licensed under *both* the UIUC and MIT license. The reason for this is that the UIUC license (like many BSD licenses) requires that binaries that link LLVM code contain a notice that they are derived from LLVM.
This clause doesn't make sense for libraries like libc++ and compiler_rt: these get implicitly added to the link line by the clang driver, so users of the clang compiler don't know that they're linking in pieces of LLVM. For example, if Mozilla was every shipped built with Clang, it should not have to have a notice saying that it has code derived from LLVM (unless they were using the JIT or something other LLVM component).
Since copyright is distributed among all the contributors, we got approval from everyone who has contributed to these two projects, and had to revert one libc++ patch (whose author did not respond to numerous requests). Going forward, contributions to these two projects will require that you allow your code to be used under either license. One bad thing about this change is that it is no longer valid to move code from a "just UIUC licensed" codebase to libc++ or compiler_rt unless the copyright owner approves, but I don't expect this to be a significant issue in practice.
I added a paragraph here to explain this, please let me know if the wording doesn't make sense: