The problem is that the function itself gets the rowdata so I can't intervene anymore. The radiobuttons/checkboxes are catched but the droplists aren't.

The bold section below is the fix for the problem.(nested if could be done better but my jquery skills are not reaching here ).

GridToForm: function(rowid, formid) {
            return this.each(function() {
                var $t = this;
                if (!$t.grid) { return; }
                var rowdata = $($t).jqGrid("getRowData", rowid);
                if (rowdata) {
                    for (var i in rowdata) {
                        if ($("[name=" + $.jgrid.jqID(i) + "]", formid).is("input:radio") || $("[name=" + $.jgrid.jqID(i) + "]", formid).is("input:checkbox")) {
                            $("[name=" + $.jgrid.jqID(i) + "]", formid).each(function() {
                                if ($(this).val() == rowdata[i]) {
                                    $(this).attr("checked", "checked");
                                } else {
                                    $(this).attr("checked", "");
                                }
                            });
                        } else {
                            // this is very slow on big table and form.
                            if ($("[name=" + $.jgrid.jqID(i) + "]", formid).is("select")) {
                                text = "option[text='" + rowdata[i] + "']";
                                $("[name=" + $.jgrid.jqID(i) + "]", formid).find(text).attr("selected", "selected");
                            } else {
                                $("[name=" + $.jgrid.jqID(i) + "]", formid).val(rowdata[i]);
                            }
                        }
                    }
                }
            });
        }