It is a paniculata introduction from Ball Horticulture. They claim it is good to -30F:

http://www.ballhort.com/Nursery/plant_info.aspx?phid=034001085023790

Other online nursery vendors either repeated that information or gave estimate to either hardy to Zone 3 or hardy to Zone 4. For example, Better Home and Gardens said Z4:

http://www.bhg.com/garden-store/flowers-plants/shrubs/hydrangea/bombshell-hydrangea.html

It is quite new so not much information is available from consumers who have tried them. But paniculatas in general do well in Zone 4. If you order by mail, be prepared to wait for shipping until either after summer temperatures recede or until Spring 2012 if they sold out.

Thanks for your reply. I did buy a couple of them and they did make it through the winter, although it was a very mild one here in Iowa this year. We'll see how well they do this summer!

Buttercupia, how are the Bombshell hydrangeas doing so far? My Lowe's has some, at a pretty steep price, but I am looking for something in their height range to replace this spirea hedge that has been bothering me for almost ten years now. It has to go, and I consider to replace it with a low hedge of Bombshell or Little Lime hydrangeas. Any feedback on how thye look, and what height they have for you would be appreciated.

Thanks so much in advance!!

Just chiming in that I am IN LOVE with Little Lime. I have 3 and plan on buying many more. They have so many blooms despite 2 of them being very very young plants. Everyone that sees them asks me what they are. Simply stunning.
I haven't seen bombshell yet and I'm interested in that too but I'm not sure if it could be any better than Little Lime.