A:
This is compatible with the iMac but you will need to purchase a iMac adapter which can be purchased at the Mac Store http://store.apple.com/us/product/MC772ZM/A?afid=p210|frgl&cid=AOS-US-SHOP-Froogle
You also might want to consider the similar MF215 which includes Virtual Axis 3D which would allow you to move the monitor 360 degrees for portrait and landscape viewing.
http://www.bestbuy.com/site/Sanus+-+Full-Motion+Mount+for+23%22+-+37%22+Flat-Panel+TVs+-+Extends+15.5%22+-+Black/8327697.p?id=1177112312815&skuId=8327697&st=MF215&cp=1&lp=1