Those antique-looking square or round rings with the "halo" around them:
http://www.bluenile.com/diamond-engagement-ring-platinum_7813http://www.overstock.com/Jewelry-Watches/14k-Gold-1ct-TDW-Diamond-Engagement-Ring-H-I-I1/1856191/product.htmlThere's a post on the Just Engaged/Proposals board where people are showing their rings, and a ton of them are in these styles. Two of my friends have these, too.
It just seems like they sprouted up in popularity all of a sudden.