there is this:
Seems to be dead. There are some examples on the site using an old version of extjs and I have seen at least one example of someone using it with a newer version but it was not cross platform.
I would like to see something in the area as well.
take a look at Origramy
Origramy (http://www.origramy.com) can produce Flash diagrams, including flow charts. I haven't tried to interface it with Ext yet, but with 3.0's Flash Charts support, the basics might already be there.