I've been using Nu with previews of iOS6 and Mountain Lion since June.
I'm not aware of any problems, although I've seen and fixed many
compiler warnings that have appeared with clang. Also, as Johannes has
pointed out, cblock bridging is not in my repo yet.
Also, you may have noticed build problems as Xcode changes have moved
files from their traditional locations into the Xcode app bundle.
Rather than try to track these, my repo now requires installation of
the Xcode command-line tools.
http://developer.apple.com/library/mac/#documentation/DeveloperTools/Conceptual/WhatsNewXcode/Articles/xcode_4_3.html
Tim