Empire: All your examples have shown is what beautiful spaces roundabouts can be if there is the will to make it happen. Safety can be managed through design. We could wall the centre so that pedestrians aren't tempted to cross traffic to get to it, we could put in underground tunnels (like at the Arc de Triomphe) or we could redesign to slow traffic so that pedestrians have some good crosswalks to use. It's never going to be 100% safe, but nothing in life is. Putting art out on the edge, seems to me, fairly pointless. Even if we end up not doing anything with Armdale because of safety concerns, I hope HRM will at least consider using the centre space in smaller more neighbourhood type roundabouts.
Just for fun, check out this "sculputure" from the centre of a major roundabout in Cardiff, UK.
http://flickriver.com/photos/richardandgill/159272903/
http://www.photoeverywhere.co.uk/britain/cardiff/slides/magic_roundabout_cardiff_jpg_orig.htm
Something simple from Shanghai that might work well at Armdale as the landscapping isn't too high to block sightlines and it's pretty, but not the kind of thing that would tempt pedestrian out to the middle if HRM wanted to keep them out of the centre. Note also the low wall that further discourages pedestrians from wandering out there without being ugly.
http://www.flickr.com/photos/martell/4825553936/in/pool-roundaboutart
Another one with a bit of road humour, the traffic light tree from Canary Wharf in London.
http://www.flickr.com/photos/squirmelia/2451382877/
Here's something simple that still allows visibility and doesn't tempt pedestrians. Again, checkout the use of materials to discourage anyone who might think to wander out there.
http://www.flickr.com/photos/20923094@N04/3657388063/in/pool-roundaboutart
Even something as small as a tree with the right materials can be beautiful as in this roundabout outside of Perth.
http://www.flickr.com/photos/martybugs/2052823536/in/pool-roundaboutart
Why is Halifax always the place that can't when the rest of the world seems to have it figured out?