Finally I found it!
For those who need default search results in ModelAdmin, this is how it can be easily achieved:

Add this method to your ModelAdmin subclass:

function getEditForm(){
	$defaultModel = 'YourDataObjectClassName';
	$collectionController = $this->getCollectionControllerClass($defaultModel);
	$collection =  new $collectionController($this, $defaultModel);	
	return $collection->ResultsForm(array());
}

Edit : this can be done with less code :

function getEditForm(){
  return $this->bindModelController('YourDataObjectClassName')->ResultsForm(array());
}

Genius. Thank you for sharing :)

Edit :( Small issue though, the "back" "Delete" and "Save" buttons do not appear when you select an object from the default list. Click search again and its fine. This is on a 2.4.0 dev server, will try on 2.4.1

Ah yes, I see it. It's because there is no history set, so the buttons will be hidden.

Not an easy fix. But let me dig into that....

Ok, this is posible to fix, but you will have to add an extra javascript file in your ModelAdmin Subclass to add a correct url to the history array.

//MyModelAdmin

function init(){
		parent::init();
		Requirements::javascript('/myfolder/javascript/MyModelAdmin.js');
	}

MyModelAdmin.js

(function($){
$(document).ready(function() {
						
	// Hide goBack and goForward buttons on initial load
	if(!this.future || !this.future.length) {
		$('#Form_EditForm_action_goForward, #Form_ResultsForm_action_goForward').hide();
	}
	if(!this.history || this.history.length <= 1) {
		$('#Form_EditForm_action_goBack, #Form_ResultsForm_action_goBack').hide();
	}						
	
	/**
	 * This will create a url with data from the corresponding SearchForm of the Default DataObject.
	 * Don't use live to ensure this is run first on initial load.
	 */
	$('#right #Form_ResultsForm tbody td a:not(.deletelink,.downloadlink)').click(function(){
		// Set history on initial load for default managed Model
		if(!this.history || this.history.length < 1) {
			var action = $('#Form_ResultsForm').attr('action').replace('ResultsForm?','SearchForm');
			var form = $('form[action="'+action+'"]');
			if(typeof $(form).attr('action') != 'undefined'){
				var url = $(form).attr('action') + '?' + $(form).serialize();
				$('#ModelAdminPanel').fn('addHistory', url);
			}
		}
	});
		
})
})(jQuery);

Please let me know if this works for you as well.

BTW, there's two tickets on that already:
http://open.silverstripe.org/ticket/3119#comment:4
http://open.silverstripe.org/ticket/5947#comment:4

Thanks Martijn! Very usefull!

Reuze bedankt hiervoor, Martijn. You rock.