I just noticed that ENV['TZ'] = nil raises an error in 1.9
 >> ENV['TZ'] = nil
TypeError: cannot assign nil; use Hash#delete instead

May I ask what is the reasoning behind this change? It's a
little troublesome for this kind of code:
   old = ENV['TZ']
   ENV['TZ'] = 'EST5EDT'
   t = Time.now
   ENV['TZ'] = old  #Exception if old == nil

which then has to be changed to something like:
   old ? (ENV['TZ'] = old) : ENV.delete('TZ')

Kinda inelegant IMHO, and prone to errors.

-Daniel