Installation of chef-server from gems now fails since rack-1.2.1 was released.
Needs a paranoid dependency check in the gemspec.
Added strace output to http://gist.github.com/643942#file_strace_chef_server.txt
 downgrading rack to version 1.2.0 got things working for me.
See also CHEF-1594. Debating gem requirements.
For the record, the bug in rack is here: https://github.com/rack/rack/issues/29
I've fixed this by applying a monkey patch to make Regexp.union work the same on 1.8.6 as it does on later versions: