BibTeX not only has a tendency (by default) to mess about with the
case of letters in your bibliography,
also makes a hash of accent commands:
"ma\~nana" comes out as "ma nana" (!). The solution is
similar: enclose the troublesome sequence in braces, as
"{\~n}", in this example.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=bibaccent