one of the documented procedures in this installation of the ACS
util_GetCheckboxValues   form   checkbox_name   { default_return "0" }
What it does:
For getting all the boxes from a set of checkboxes in a form. This procedure takes the complete ns_conn form and returns a list of checkbox values. It returns 0 if none are found (or some other default return value if specified).
Defined in: /web/philip/packages/acs-core/utilities-procs.tcl

Source code:

    set i 0
    set size [ns_set size $form]

    while {$i<$size} {

	if { [ns_set key $form $i] == $checkbox_name} {

	    # LIST_TO_RETURN will be created if it doesn't exist

	    lappend list_to_return [ns_set value $form $i]

	incr i

    #if no list, you can specify a default return
    #default default is 0

    if { [info exists list_to_return] } { return $list_to_return } else {return $default_return}