Okay - I've taken a few more shots (50mm, 75mm, 135mm at f2.8, f4, f5.6 and f8 - total 12 pictures) in a consistent environment
camera on tripod, 2s delay shot - to eliminate motion blur
lens focused using Live View, and then focus locked - so no focus variation across shots
the focus is on the fur just below the bear's nose
ISO 100
Here's the link to the album containing the shots (also includes pictures from original post):
DA50-135 focus - Windows Live
My conclusion?
I think Adam is right - the lens is soft wide open - not too bad at 50mm but progressively worse at longer focal lengths
Quite sharp at f4 at 50mm
Needs f5.6 at 75mm
Really needs f8 at 135mm
Can someone else who owns the lens do a similar test and post their observations?
I'm kind of disappointed since I bought this lens for sports photography, and I don't really want to shoot at f8 all the time at 135mm to maximise sharpness.