Bach states the following: mkdir is a typical setuid program. Only a process with Effective UID root can create a folder. (Section 7.6, Page 229) I checked the permissions on it on Ubuntu ...
Sometimes you have a stuck process that's been stuck for a while, and as soon as you go to poke at it with strace/truss just to see what's going on, it gets magically unstuck and continues to run! So ...