1. open a any image
2. use select rectangular region tool to pick an image region
3. press ctrl-c to copy
4. press ctrl-v to paste
5. in the layers window a new layer appears called 'Floating Selection' (pasted
layer)
6. at this point, dragging the selection can move it around, clicking outside
the selection merges it with the layer below it. 
7. until this 'merging' is done, another layer can't be selected even though
other tools cann be

for me, this interaction is confusing and a little bit frustrating, especially
not being able to change layers and not really being sure why. 

Here are some suggestions borrowed from other graphics packages that handle this
interaction a little more intuitively:

possibility 1:
always paste into a new layer (no intermediate layer) and let the user decide if
and when he wants to merge the layer with another layer

possibility 2:
paste into the currently selected layer (with the intermediate layer produced as
is currently the case) but merge floating selection with current layer (possibly
preserving the selection marquee) whenever another layer is selected or when the
tool is changed.