Typically in my code, I open and select a database in a prepended file, and use that same database throughout my entire page.  I've also been building some modular plugins to my site that may require a different database selected.  It'd be nice if mysql_select_db() would return the currently selected db so that in a plugin module I could do something like this:

$OldDB = mysql_select_db( "SomeDB", $dbConnection );

/*
    bunch of other code
*/

// return to main site
mysql_select_db( $OldDB, $dbConnection );