I'm not sure that completely ignoring unrecognised text files is the best form of DWIM. It took me several searches to realize that ack does not recognise TeX files. I suspect that this would mean that extensionless plain text files would not be searched since they have no #! line. Having to specify the --all option to search these files takes away some of the fun since then backup (~) files are included then. Perhaps a --text option (anything matching -T that is not a backup file) as a counterpart to the --binary option would be nice.