To blur props you'll need at the minimum 1/60th shutter speed, the slower the better. A lens with IS is helpful here.
Exactly true on slower shutter speed. I like to show a little definition of the blades as opposed to just blur and find any shutter speed at and under 1/200 sec works well in full daylight. But, get too slow in daylight and you might blow out the exposure.
As examples, this Avantair P180 (link)
I snapped at our local airport used a shutter speed of 1/125th sec. (.008 sec) and the prop blur came out perfect for what I was going for using a "faster" slow shutter speed. This allowed me to show the P180's 5-bladed props and get a nice effect.
This panning shot of a Columbia 400 departing SQL (link)
which used 1/50 sec (.02 sec) and allowed more background blur and greater prop blur - so much so on the latter that the prop blades are no longer individually discernible.