As far as I can tell IB3.0 still needs some work. It has a much more intuitive way to make connections between objects. The new method uses aperture like floating panes (opened via right click or ctrl-click) listing actions and outlets, however I found that selecting an item from the list is quite buggy and often misconnects to the wrong item. The workaround I found was to connect from the pane to the object instead of from the object to the pane. Note: These new floating panes are separate from the inspector, the inspector still shows connections and outlets like in the Tiger IB.
It is also apparent that IB3.0 has not been tied in with Xcode 3.0 yet. An example of this is how when you open a Nib via Xcode 3 it will not show images from the Xcode project, like the older Tiger version does. Also, as far as I could tell the default IB was still the Tiger version (which is included in Xcode 3).
Edit: Some more stuff:
Selected objects now have an added blue highlight along with the bounds.
Resize bounds have been simplified, for example a round button has no bounds, a push button only has them on left and right and a gradient type has them on all sides and corners. So basically bounds respect the fact that certain objects can only be resized from certain points.
It is now possible to make connections via right click, from what I can tell only ctrl+click works on the Tiger IB.
The default NSApplication object no longer has basic actions like hide and terminate which you could see in the Tiger IB, although both IB have those actions in First Responder.
The inspector has a really cool way of viewing info from different objects quickly. Simply click and drag the white cog on the top left over any object in your IB project to see it's info in the inspector.