This is a question rather than a feature request.

This is how I accomplished:
I want to expand the node with id 123
var li = $('#123', tree); //tree stores a reference to the root node
var liElem = li[0]; //search returns an array even if only one element found
//find the child of this node
var childUI = $('>ul', liElem);
if(childUl.is(':visible')){ //allready expanded
//do something here
} else {
tree.nodeToggle(liElem);
}

If you don't care if the node is expanded or not
tree.nodeToggle($('#123', tree)[0]);

I found the solution.

var simpleTreeCollection;
$(document).ready(function(){
simpleTreeCollection = $('.simpleTree').simpleTree({
autoclose: true,
drag: false,
afterClick:function(node){
//alert(node.attr('id'));
//alert($('span:first', node).text());
var loc = $('a:first', $('span:first', node)).attr('href');
window.location=loc;
//alert("text-"+$('span:first',node).text());
},
afterDblClick:function(node){
//alert("text-"+$('span:first',node).text());
},
afterMove:function(destination, source, pos){
//alert("destination-"+destination.attr('id')+" source-"+source.attr('id')+" pos-"+pos);
},
afterAjax:function()
{
//alert('Loaded');
},
animate:true,
docToFolderConvert:true
});

expand();
});

function expand()
{
simpleTreeCollection[0].nodeToggle($(expandnodeid, simpleTreeCollection[0])[0]);
}

from codebehind (since I use ASP.Net, I set the var expandnodeid). This works perfectly.

It isn't even necessary to have a reference to the rootnode. All you need to know is that simpletreecollection is a collection of multiple simpletree objects on the site. If you have one simpletree you call simpletreecollection[0]. This wasn't clear, but I figured it out.