Interesting that they're moving so fast to depend on C++11 support, rather the opposite of the GCC philosophy (which AIUI keeps a stage1 written in K&R C to ensure that it can build on any platform). Could it be that even after clang spreads, we'll need to keep gcc around for bootstrapping?

Bootstrapping from the previous version of clang would be easier. GCC cares so much about portability because the GNU project's goal was to replace the userspace on proprietary Unices, however ancient or quirky. There are few such systems left, and they are not clang's target audience.