ListStore store=combo.getStore();
store.setModelComparer(new ModelComparer(){
    .....
});

BaseModel model = new BaseModel();
model.set("id", 1);
int idx=store.indefOf(model);
combo.select(idx==-1?0:idx);

package test;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.Widget;
import com.sencha.gxt.data.shared.LabelProvider;
import com.sencha.gxt.data.shared.ListStore;
import com.sencha.gxt.data.shared.ModelKeyProvider;
import com.sencha.gxt.widget.core.client.button.TextButton;
import com.sencha.gxt.widget.core.client.container.VerticalLayoutContainer;
import com.sencha.gxt.widget.core.client.event.SelectEvent;
import com.sencha.gxt.widget.core.client.event.SelectEvent.SelectHandler;
import com.sencha.gxt.widget.core.client.form.ComboBox;

public class TestEntryPoint implements EntryPoint {

	public void onModuleLoad() {
		RootPanel.get().add(asWidget());
	}

	static class Bean {
		int id;
		String label;

		public Bean(int id, String label) {
			this.id = id;
			this.label = label;
		}

		public Bean(int id) {
			this.id = id;
		}
	}

	private Widget asWidget() {
		VerticalLayoutContainer vc = new VerticalLayoutContainer();

		ListStore<Bean> store = new ListStore<Bean>(
				new ModelKeyProvider<Bean>() {
					public String getKey(Bean item) {
						return String.valueOf(item.id);
					};
				});
		store.add(new Bean(0, "00"));
		store.add(new Bean(1, "11"));
		store.add(new Bean(2, "22"));
		store.add(new Bean(3, "33"));
		store.add(new Bean(4, "44"));
		store.add(new Bean(5, "55"));

		final ComboBox<Bean> cmb = new ComboBox<Bean>(store,
				new LabelProvider<Bean>() {
					public String getLabel(Bean item) {
						return item.label;
					}
				});

		vc.add(cmb);

		SelectHandler sh = new SelectHandler() {
			public void onSelect(SelectEvent event) {
				ListStore<Bean> store = cmb.getStore();
				Bean b = store
						.findModelWithKey(((TextButton) event.getSource())
								.getText());
				cmb.setValue(b);
			}
		};
		TextButton b0 = new TextButton("0");
		b0.addSelectHandler(sh);
		vc.add(b0);

		TextButton b1 = new TextButton("1");
		b1.addSelectHandler(sh);
		vc.add(b1);

		TextButton b2 = new TextButton("2");
		b2.addSelectHandler(sh);
		vc.add(b2);

		TextButton b3 = new TextButton("3");
		b3.addSelectHandler(sh);
		vc.add(b3);

		TextButton b4 = new TextButton("4");
		b4.addSelectHandler(sh);
		vc.add(b4);

		TextButton b5 = new TextButton("5");
		b5.addSelectHandler(sh);
		vc.add(b5);

		return vc;
	}

}