The binary substituter can use several threads (currently up to the number of CPU cores). I saw it segfault randomly (obviously after the release...) and found out that this was caused by Guile’s ‘regexp-exec’, which is not thread-safe as of 2.0.9:
http://bugs.gnu.org/14404 Commit 0332386 works around that. Just run “guix pull”, and you’ll be safe. :-) Ludo’.
