time_zone_select() Trouble

I am having a lot of trouble getting my time zone select to default to
the user's previously selected time zone. Here's my code:

<%= time_zone_select :other, :time_zone,
  TimeZone.us_zones,
  :selected => cookies[:time_zone] %>

This didn't work, so I checked all my cookie action (which seems fine)
and finally tried the following just to see if it was working at all:

<%= time_zone_select :other, :time_zone,
  TimeZone.us_zones,
  :selected => "Hawaii" %>

That doesn't work either. Any ideas?

Here's what the API says about it:
http://api.rubyonrails.org/classes/ActionView/Helpers/FormOptionsHelper.html#M000943