The bitcoin wallet is user data and so should go in: $XDG_DATA_HOME/bitcoin/ The block chain is cache, and can be reloaded from the internet and so should go into $XDG_CACHE_HOME/bitcoin/ The configuration file is config and so should go in $XDG_CONFIG_HOME/bitcoin/
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html