Found on the clang development mailing list, cfe-dev, in the middle of debugging a build failure, from someone with an apple.com mail address:
I build on [Mac OS X] 10.6 and it usually works just fine. Building on 10.5 wasn’t much different.
That 10.6 is under development is hopefully without question, but that it’s so far along (or indeed perhaps so minor) that people are already living under it is intriguing.