Converting to Secret Language
Smalltalk (Smalltalk/X), 29 27 chars
I am lucky - it is already in the String class:
'Today is a great day' rot:2
    -&gt; 'Vqfca ku c itgcv fca'

adding I/O, this makes it:
(Stdin nextLine rot:2)print

in the spirit of doorknob's obfuscated example below, how about:
Parser evaluate:('(Uvfkp pgzvNkpg tqv:2)rtkpvPN' rot:-2)