It appears that the changes made in SVN revision 2134 to Ext.form.Field are causing an empty ComboBox's value to be set to the emptyText value. I didn't look too much into the code that seems to have caused this, but I believe it's a change made to the afterRender method.

Here's some simple code to reproduce the bahavior I'm experiencing, although I'm sure it could be duplicated using any example with a TextField (or any component derived from Field) that has the emptyText property defined.

Code:
Ext.onReady(function(){
Ext.QuickTips.init(); 
 
var combo = new Ext.form.ComboBox({
 id: 'testCombo',
 emptyText: 'Select One...',
 typeAhead: true,
 triggerAction: 'all',
 mode: 'local',
 store: new Ext.data.SimpleStore({
     fields: [
         {name: 'id'},
         {name: 'name'}
     ],
     data : [
         [1, 'Chevrolet'],
         [2, 'Volkswagen'],
         [3, 'Honda'],
         [4, 'Ford']
     ],
     sortInfo: {field: 'name', direction: 'ASC'}
 }),
 displayField: 'name',
 valueField: 'id',
 forceSelection: true,
 allowBlank: false,
 renderTo: document.body
});
 
 
var button = new Ext.Button({
 text: 'Get Value',
 renderTo: document.body,
 handler: function() {
     Ext.MessageBox.show({
         title: 'Hello!',
         msg: 'The value of the combobox is \'' + Ext.getCmp('testCombo').getValue() + '\'',
         icon: Ext.MessageBox.INFO,
         buttons: Ext.MessageBox.OK,
         animEl: 'testCombo'
     });
 }
});
 
});
Running the above with the SVN build, both Ext.getCmp('testCombo').value and Ext.getCmp('testCombo').getValue() return 'Select One...'. The same in Ext 2.1 returns ''.

Please let me know if you need more info or if I can be of help.

-Rocco