Hi there,
the babel package delays the activation of its special characters, the so-called shorthands, to the begin of the document not to confuse other packages, IIRC.
To activate one of them in the preamble, you can use \shorthandon/\shorthandoff, i.e.,
well, the \shorthandon/\shorthandoff macros aren't limited to single characters.
Perhaps babel's option `KeepShorthandsActive' does what you want, too.