in the current stable (0.9.0) thats used in ubuntu intrepid, i get this:
there is always one item selected (even at startup).
if you click on that item the tree gets the keyboard focus and you can navigate up/down etc. inside.
if you click on one of the unselected items, that item becomes selected and the focus changes to the main view.
the difference is quite annoying for me. i dont think its intended so im reporting it.

i would expect the following behaviour:
no matter what is currently selected in the tree pane, if you click on an item it gets selected and the keyboard focus gets there (e.g. if you press "up", you select the item above the one you clicked before).

(if the current behaviour is intended to provide a "faster" way to get the focus inside the mainview of another folder,) i would propose to use a double click for that: e.g. if you double click on a folder, the item gets selected, if there are subdirs, it gets expanded and the focus changes to the main view showing the clicked folder.

i just discovered, that double clicking does exactly what i was expecting for normal clicking.
so id argue:
clicking once is normaly selecting an item, double clicking is opening/executing/doing something actively with it.
translated to the tree view this would correspond to what i proposed above:
single click: selecting, "peeking" into the folder == no focus change
double click: selecting, expanding the subdirs and "entering" the directory by changing focus.

subtle difference, but inconsistencies (first half of this report) are bad bad bad gui design.