Hi I'm using comboboxes as in this example to filter do a query to server and usually it returns a lot of results.
Initially I supposed keys pageUp an pageDown where mapped to move over the combobox list, but onli up and down where mapped so, I added them to the code.
It's quite simple and maybe I missed some details, but it works.
The code is added to ext-all.js (I added to ext-all-debug.js by comodity) to method initEvents in ComboBox definition:
Code:
        Ext.form.ComboBox.superclass.initEvents.call(this);

        this.pagination=this.pageSize!=0?this.pageSize:19;

        this.keyNav = new Ext.KeyNav(this.el, {
            "up" : function(e){
                this.inKeyMode = true;
                this.selectPrev();
            },

            "down" : function(e){
                if(!this.isExpanded()){
                    this.onTriggerClick();
                }else{
                    this.inKeyMode = true;
                    this.selectNext();
                }
            },
            "pageUp" : function(e){
                this.inKeyMode = true;
                for(i=0;i<this.pagination;i++){
                    this.selectPrev();
                }
            },

            "pageDown" : function(e){
                    this.inKeyMode = true;
                for(i=0;i<this.pagination;i++){
                    this.selectNext();
                }
            },

            "enter" : function(e){
                this.onViewClick();
                            },

            "esc" : function(e){
                this.collapse();
            },

            "tab" : function(e){
                this.onViewClick(false);
                return true;
            },

            scope : this,

            doRelay : function(foo, bar, hname){
                if(hname == 'down' || this.scope.isExpanded()){
                   return Ext.KeyNav.prototype.doRelay.apply(this, arguments);
                }
                return true;
            },

            forceKeyDown : true
        });