bordersplitter as a child component
One difference I noticed just now is that in my border layout with a west panel (with split: true) and my center panel the resizer splitter is a child component of the parent container with the border layout.
In 4.0.7 I would use westpanel.nextSibling() and would get the center panel. Now doing that I get the Ext.resizer.BorderSplitter.
Not a bad thing really, but was something that I had to refactor to get 4.1 beta 1 to work in my particular app / layout so more just FYI to peers.