DIO stands for "Database Interface Objects" and is shipped with Apache Rivet. It is a database independant access layer that lets you utilize different databases with the same code. It was written by Damon Courtney.
More info here: http://tcl.apache.org/rivet/manual/dio_package.html
NEM: Link broken (19Dec2005). Anyone know where DIO docs live now?
schlenk: They are inside the the rivet svn. (http://svn.apache.org/repos/asf/tcl/rivet/trunk )
Massimo Manghi: Rivet is available at http://www.apache.org/dyn/closer.cgi/tcl/rivet and can be downloaded also as binary for SuSE from http://software.opensuse.org/search?q=rivet