Argh I can't believe how long it took for me to track this one down. No one else on the internet had reported the problem before, so trusty ole google was no help. So I'm putting this one out there, to help others.
This error is caused when a file vendor/rails/activesupport/lib/active_support/values/unicode_tables.dat becomes corrupted. The easiest way that can happen is if while using FTP to upload your Rails app onto it's server, you are uploading the entire vendor/rails directory in ASCII mode. unicode_tables.dat contains binary data, which gets mangled when you upload using ASCII. Read more